After editing a poly mesh with a ZBrush modeling tool it managed to split the UV of a group into two islands. So far I've tried a couple of methods thinkable both with Houdini as well as Blender. This seems so weird.

Do you know how to unite the UV islands without redoing the unwrap?

In this case the UV:s of the region look unwrapped, but they are messed up. One solution to such a situation is to unwrap only the particular area and match it back into the UV island (group). For me this is easiest with Blender's UV tools.

You can sew UV seams with UV Flatten SOP, but if you want to work on existing seams, it's quite a chore to feed them into the operator. See UV Flatten: Sew UVs with pre-existing seams [www.sidefx.com] for more information about this subject.