combining diacritics

[Bèrto ëd Sèra]

Hi all!

I'm dealing with minority languages and I often bump into charachters
that are missing from Unicode as single codepoints, but can be easily
'built" by using combining diacritics. I cannot seem to find a way to
assign them to keys (as ready made chars), though.

I can easily build a dead key, but this is fairly complex to use,
especially since often the diacritic is not present on the keyboard as
such, only the base char is.  So end users have to guess where the
diacritic is, while having a 3rd level definition with AltGraf placed
on the base char would make for a fairly natural way to access it.

Yet, no matter how hard I looked for it, I could not manage to find a
way to assign a sequence of symbols (like an HTML ö̴̼́)
to a key in a symbol file.

Is it impossible, or is it me being dumb?
Berto