Hroe,
You don't need to copy & paste anything. Your images have IPTC data. Canstock reads them and add suggested new keywords at the end, you select that choice.
If you intend to copy those keywords to the other sites, my suggestion is to upload to CanStockPhoto first, see the suggestions and apply them to the IPTC data of your files before submitting them to the other agencies.
In my case I never had a crucial new lkeyword added, although there were a few interesting ideas. And what CanStockPhoto considers a suggestion may be consider a spam (maybe a stretch) in other sites.
Regards,
Adelaide