Hi niko, to do an "untrim" first use Edit > Separate to break out the surface you want to untrim so it isn't joined to other surfaces. Then select all the edges and hit Delete, that will remove the current trim boundaries and recover the full underlying surface. A quick way to select all edges of a surface is to first select just one edge and then do Ctrl+A.

There's more info on untrimming here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4

And also in the object repair tutorial here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

- Michael