Instantiate and combine mesh without creating new gameobject

I'm making a game where I have a 3D tile engine, where the blocks are generated procedural as one mesh. Now I want to make some nifty tile transition, but there is one problem:

What I want to do is to instantiate and move the premade models of corners/walls, without making a new gameobject for every new tile, and then combine them somehow into one supermesh. How do I do that?

I know about Mesh.CombineMeshes, but what I don't know is how to instantiate meshes, and manipulate them.

Thanks in advance!