Legacy GM Replacing a sprite on a texture page

I'm working on a dungeon crawl-esque game, and I'd like to allow users to create entirely new content for it. To do that effectively, they need to be able to import sprites for their new items, enemies, terrain, et cetera. Terrain, though, poses a problem, since I'm using a primitive to render it all as one "object" to save on GPU cycles: Anything new cannot be rendered in the same chunk as anything from a different content package, be it the base game or another mod, since they would occupy different texture pages.

Is there any way to replace an existing image on a texture page with a new one? That would allow me to "reserve" large amounts of the texture page for new images by creating blank squares, and the game would swap those blank spots out with user-created content while loading.