[SPAMKLASSAT av SU] Writing strange diacritics

[ezzetabi]

I need to writing voels with strange diacritcs. E.g.: ā,á,ǎ,à,ü (copied 
and pasted from somewhere...)

I configured the left Windows Flag of my keyboard for being Multi_Key, 
but the default combos aren't enough, since I can't (or did not find 
how) writing carons and macron.

How can I configure new Multi_key combos?

I know it is not XFCE directly related, but I can't find anything in the 
Internet. So I hope anyone can help me.
Thanks.