Is it possible to use additional key to show that a amenity=parking that is reserved for staff during working hours can be used by the generel public (for a fee) outside of that (e.g. mon-sun 18-06)?
That should do the magic:
access=private
access:conditional=yes @ Mo-Su 18:00-06:00
fee=no
fee:conditional=yes @ Mo-Su 18:00-06:00
Check out the wiki for more information: Conditional restrictions - OpenStreetMap Wiki
In general, simply fee=yes would also suffice. Employees will certainly not get this information from OSM 
Too long for just a comment 
Please ask as many questions as you like, that’s what the forum is for
The bracket is not quite correct in this case, it should be:
access:conditional=yes @ (Mo-Fr 17:00-07:00; Sa-Su)
But in this case, however, Monday morning will always be missing, so we have to specify the individual time ranges individually so that each day really works.
access:conditional=yes @ (Mo-Fr 00:00-07:00,17:00-24:00; Sa-Su 00:00-24:00)
You can view the result here: opening_hours evaluation tool