A dialog for File->Merge

[micheus]

Preserve selection: it will preserve the selection of your current scene (if there is any one)
Add to current: the selections present in file being merged will be added to the current scene selection (using that current mode)
Replace current: what the regular merge command do today - replaces the selections in the current scene with those ones present in the scene being merged.

You can download the update in order to check it better.
There is only a small issue that I'm working on that is about the "Add to current" option: the Geometry window isn't updated with the right selection. You can see in the Geometry Graph the previous selected objects with the icon in red, then if you hide/show (eye icon) the object it fix the visual issue. (it's ok now)