mixing uv's

I want to specify some uv coords before I collapse my mirror modifier, so that part of the mesh share the same coords and the rest after the collapse for parts where I dont want shared
I can do it in max, but I don’t know how in blender or if it is possible

Unwrap, rearrange your UV map (see wiki topic on Unwrapping your Mesh) and then PIN the UVs you want to not move. Then unwrap the rest of the faces.

Note that the new seamed unwrap uses the LSCM, all but eliminating the need for multiple unwraps in most cases. njoy.