@michael_oborne
thank you very much for the clarification!
Obviously a case of mishandling data by me, the user… Thanks for handling this correctly in Mission Planner!
I indeed have the decimal indicator regional setting set as “comma” on the computer I am working on now.
As I thought about your reply and me entering the altitude data with a period instead, my mishandling must have stemmed from thus far primarily using a client’s computer for generating missions and WP files - which has different regional settings. So I was very used to entering data into WP and home fields in MP (and into Excel files used to generate WP lists) with periods. Compounding to this mix-up: Google maps does not understand coordinates unless they are entered using a period, even with my local regional settings set to “comma” (!).
So thankfully this issue is cleared up and found to be possible without needing a workaround - and a few more facets have been learned 
(Note that in the “Default Alt” box, though, it is not possible to enter fractional values.)
Now, to complete the flight plan creation for this application, I need to mix both relative and terrain-based WPs.
You had mentioned previously, that MP currently only supports a single mission frame per mission. (I.e. on saving the flight plan, all WPs are “equalized” to one type of altitude reference - even those that previously were entered while in another reference scheme.)
If I now manually amend/ create a flight plan, with having either “3” or “10” as frame in the third column of a WP file, as needed, and open such a mission file in MP, it is being completely “erased”, with not even coordinates present anymore…
Is there another way to upload such a “mixed-altitude-reference” WP-file from within Mission Planner or otherwise?