That is not permitted by the terms of some microstock agencies, and others require an enhanced license of some sort to be allowed to produce print-on-demand items.

I know Dreamstime will allow it with an Enhanced License, although I have opted out of that because I put my own artwork up on Zazzle, I don't need someone buying an EL to compete with me with my own artwork.  Someone just requested an EL at Dreamstime, for one of my images, and I declined it because they had said they would be using it for print-on-demand.

Be sure to read the Terms of Service for each agency where you have your images, and find out how to opt out, if this is something you don't want happening.

Good luck.

Stacey