Philippe Noth

I would say the "best practice" is to be sent a properly prepared sdlxliff in de(DE) - fr(CH).  But given you have not here's a few ideas and there may be more:

Change the language codes in the sdlxliff: Easy to do with a text editor and you can open the sdlxliff in Studio using the correct resources.  Once complete either send your client the sdlxliff as it is or just revert the language codes so they get what they provided you.

Use AnyTM: Open the sdlxliff in your de(DE) - fr(FR) project and add your TM using AnyTM.  The downside of this method is that if there are differences that affect the way the TM should work for fr(CH) then you won't get them.

I would say this is strange request from your client.  It might have been more sensible if they didn't know how to deal with this just to send you the source file and the sdlxliff.  Then you could pre-translate the source file from the sdlxliff (update to your de(DE) - fr(CH) TM using AnyTM) and deliver the reviewed target translation and an sdlxliff in the right languages.

Thanks Paul, will then use the quick and dirty tag change in the files.