Hello Idegix,
As far as I can recall (don't work there any more) nationality is not mentioned as a requirement for employment with SMATSA (Serbia and Montenegro Air Traffic Services Agency). Actually, only the DG himself has to have Serbian nationality, so that is not an issue. However, I believe you'll have to wait for the new aviation law, since the old one (still in force) does not recognize neither ratings, nor endorsements, only the "valid license". I've checked the draft version of the new law and it says that successfully finished training in a foreign training center "might be recognized, if such a training match the training that exists in Republic of Serbia". Still, even more important, then the law, will be podzakonski akt (jbg, ne znam engleski naziv) "Personal licensing", which will determine all the requirements for issuing a license, with rating and endorsement.
Don't want to be negative on the issue, but, I have my doubts on successfulness. Anyway, if that's something you really want, go for it! I'll try to dig some more information and will PM you as soon as I get them.
All the best
S.