Is there a way I can merge tilemaps into one tilemap?

I'm trying to think of a way to implement a system into my top down 2d game that instantiates prefab tile chunks modularly during runtime.

Right now I want to be able to create a handful of unique 20x20 tilemap prefab grass chunks and then allow them to populate lets say a 8x8 grid of those chunks randomly during runtime. Similar mechanism to Spelunky's randomly generated modular rooms. I would like to be able to instantiate these grass prefabs during runtime, move them to their correct transform and merge them into one tilemap, ultimately connecting the 4 grids you see in the scene into one big grid in this example (grass in an autotile). Is this even possible? If so what's the best way to implement this? I'm aware I can create this effect without using the unity tilemap system through varius tutorials but I really want to use my animated and auto rule tiles I spent way too long creating for this system.