1. Can i add FAA-IFR or FAA-ME rating to such FAA-PPL (non-standalone)?
Yes and Yes.
You do not need to convert the ME rating, you can fly a ME G reg on an FAA certificate with ME rating.
I think I'm right in saying that with an FAA certificate with IR and ME ratings you can legally fly a G registered SE and ME aeroplane, by day or by night, out of sight of the surface anywhere, and in IMC in accordance with IFR outside of controlled airspace. This actually runs worldwide, but local restrictions may prevent this.
Note: this applies to G registered aeroplanes, depending on where you're from, there may be other rules (for better or worse).