Oh, I'll try to specify it. Actually it is pretty simple:
E.g I have multiple .capx files which I would like to join together into a new .capx (with all objects, behaviours, families, variables, events, event sheets, layouts).
There should be an interface, where I could choose what objects/variables should be merged into the new .capx (e.g. select all or check boxes) and from what source .capx.
Actually this functionality is already available to some extend by copy/pasting, but as I said before you can't copy paste families.