I am no authority on this but will have a go
1. Can i add FAA-IFR or FAA-ME rating to such FAA-PPL (non-standalone)?
Yes to the 1st one (very common). Don't know on the 2nd one (don't have an ME).
2. If yes, how complicated it is to convert FAA-IFR and FAA-ME to same JAA ratings?
To convert an FAA IR to a JAA IR, you need a min of 15hrs' flight training (i.e. the 50/55 min hrs JAA flight requirement - SE/ME - is reduced to 15hrs), the mandatory JAA ground school attendance now becomes at the discretion of the school, and you have to sit all the JAA exams that are appropriate to the PPL, CPL or whatever license. I believe the min is about 8 exams. It's about 1-2 years' study if you have a job as well, etc.
An FAA IR needs an N-reg plane for simple undisputed worldwide IFR privileges, but you can get IFR privileges in other countries' registrations, with various limitations (incl. none at all) according to the state of registry. In a G-reg, an FAA PPL/IR gives you worldwide VFR/IFR privileges but IFR has to be outside CAS. There are one or two very narrow cases where an FAA IR can be used without limits in a European reg but they attach conditions like (e.g. France) you must prove your residency is outside of the EU. I have refs for the above; PM me if you want me to dig it out.
Some people have found that an FAA IR is a useful route to a JAA IR; this is of interest if you either can't get an N-reg plane, or want to comprehensively insure against whatever EASA will do with foreign-reg planes in years to come, or are looking for a commercial flying career in Europe. Otherwise, there is no point in doing it.