Yes, you can. To 20 or more if you like!  
But you have to realize that acceptance and rejections are different to the sites, so don't take that personal.
Strange enough the images that I have accepted by all the sites I uploaded them to, are mostly not my best images in my own opinion!  
Sometimes bestsellers on one site are not accepted at others or have no downloads there.
When you are going to upload to more sites you will propably discover that some sites will do better for you than others. Depending on your portfolio. So after a while it will be possible that you will decide to upload only to that stocksites.
To me Istock and Fotolia are not the easiest to get images accepted and Crestock is the worst. Fotolia does a lot of "overabundant" rejection reasons. At least to me!
Good luck!

PS: As reaction to alsfoodtown: You can upload single images exclusive to Fotolia or Dreamstime. That single images you can not upload to others of course...

Yes, I have problems with uploading with FTP to Fotolia too. For a few days now.
I think I will try the flash uploader for a while. Never had problems before, but "never"is not so very long in my case....

Flemishdreams: There's nothing wrong with being paranoid to the internet these days! I understand your point and do agree. I know something about computers and programs, but not enough to be really sure that I not could be misused. And I think most people on the internet can't. But if you do not want to take that risk: stay out of the internet. If you want to be connected, you have to take risks. And you are right to be careful...
It is difficult to know for sure if people are really to be trusted.
For what my opinion is worth: I think Alex Bakulin is simply a man who thinks there's a hole in the market for a kind of program like Cushy Stock, he wrote one and is working on it to make it better. And is hoping to make a living out of it. A lot of people do things like that (even stockphotographers ) and it is allways difficult to get a place in the market.
It is not realistic to hold one person in a country responsable for the things the government of his country does. I don't know where you come from, but in that case: don't trust any american citizen, for USA also does things a lot of people in the world don't agree with! And USA is a democracy, Russia isn't, so people there have less influence on their gouvernment than americans have. Of course also in democracies things are not what they ought to be, but I hope you get my point...
CushyStock doesn't use an internetconnection. Only for FTP-upload and for getting financial status from the stocks. And that are connections you have to made by yourself. There is never an automatic connection made as far as I can see. There's a warning in the program when you make logs of FTP-uploads that usernames and passwords are in in it. I once need to send Alex a part of an FTP-log (because of a bug) and before I removed that info from the log.
I don't know anything about the people who made Lookstat and I have to put my personal info on a website? How can I know that they can be trusted? It is allways a choice you have to make and possibly you sometimes make wrong ones. But I don't want to stay in my bed all day to avoid making mistakes...
But you're longer in this "stocky-thing" than I am, so I keep your opinion in mind. 

I am new at stock too and that also only for a few months. So there's lots and lots to learn!
I do agree with Paula about Cushystock.
I found myself in the same trouble!
When I read this: http://www.perrush.be/SYF_micro_E_1.html, it was the first time I even heard about microstock and that also non-professional photographers could get into that world. (You are now asking yourself in what cave I have lived? Well laugh loudly if you wish! ) Off course I knew about Getty's and so, but details about it lay far over my horizon.
So I found also this site, read and read and read and started with making photo's ready for upload and registering to a few sites. I am not familiar with Excel and then I found Cushystock.
Because I am from the Netherlands and there was no translation. I translated it and get a license key for free. What I found a friendly geste (or is it gesture? excuse for my bad english!) from Alex Bakulin, who is the maker.
The program is new (not even version 2) and the maker is doing hard work on it and likes to get reports about problems and bugs that are found and suggestions to make it a better program.
I am not payed by him too, by the way, but sometimes corresponding by e-mail about troubles I found! 
As far as I can see, it is a program with potention. If more people start using it and are going to report bugs, it will become a better program for me too, so it is part selfinterest so to say...
I read about Lookstat too, but I also read that people don't like to get all their personal info on a website. Don't know if they are right and if it really works that way, I didn't try it myself.
And you can simply translate Cushy in your own language when that works best for you. (and get a license key if you send the translation to them if this language is not already translated .)
Reading this site for months and learning a lot, I will even do in the future and hope and expect to learn a lot more! At least it is good for my english!
Thank you all for sharing your experience...One day I hope to reach a level to do the same for others.