Hi Jan,

You can use UV Mapper but I've found Carrara's UV toolset to be reasonably capable and that way you can skip importing/exporting geometry etc. 

With Carrara's UV you get two windows: Geometry and UV map. You can paint/select the polys in either window so making the selection is no big deal. You save the selection by name so you can recall it later to unwrap it or move it around. Not being able to see your shape, I can't recommend the best unwrap function. But do know that you can unwrap/display different selections with different unwrap modes and then scale, rotate and move them as needed. 

Depending on the detail required, you may want to designate different shading domains so you can create a larger UV map specifically for detail areas. Shading domains and different UV maps walk hand-in-hand.