Welcome to the forum BkA.
Thanks for providing an example. It makes questions so much easier to answer.
Some ideas that quickly came to mind were:
* Find a font that already had the characters you need (might be tricky);
* Layer two text frames over each other (difficult to get the correct alignments);
* Draw the arrows manually (a real pain).
I can't think of any good Scribus-only solution that would do it, however you can probably get what you need by using LaTeX - or some derivation - in a Scribus render frame.
I'm not a LaTeX expert in any way so I can't really give much extra information on this but have a look at this StackExchange posting:
https://tex.stackexchange.com/questions/159291/multiple-diacritics-on-one-character (The "stackengine" approach looks like a pretty good solution.)
It means you have something extra to learn but it might be quicker in the long run than a pure Scribus-only approach.
You would also need to be using a font that had both up and down arrows but maybe a caron or a breve would do just as well (both of which are in Arial, characters U+032C and after).
Having said all of that, maybe there's a much easier solution I just haven't thought of. Someone else might know better but this could be good enough for what you need.