My question is can you tie in at a junction box 12 gauge thnn wire to 12/2 romex? The romex was ran from the panel box. The thnn is going to be run inside 1/2 inch emt to box where I can use as a plugin

Yes, there should be no problem.

NMB (Romex is a trade name) is THHN insulated conductors, however, because they are not so labeled along the entire length of the conductor they cannot be used to take advantage of the 90(C) ampacitiy for adjustment and correction.

NMB conductors need to be sized according to the 60 (C) column. In this instance, the maximum ampacity the cable can be used for is 20 amps. Any adjustment or correction would have to start from there.