야 임마 is wrong... 야 인마 is correct... 야 인마 is 야 + 인마 '야!' is the way of calling(more like shouting not because some is far, but you are angry with someone) someone .. but for teenagers, it just the way of calling their friends 인마 is short form of '이놈아' and you can check '이놈아' in your dictionary. 이놈아 is the second person pronoun for man, and you underestimate him 이년아 is for woman.. but, you'd better not use these words, since you are old enough

I'd say 임마 is OK in informal writing. You don't necessarily have to be upset in order to use 야. But 임마 or 야 임마! sounds angry. 놈 means fella/bloke. But I've seen men used this kind of words casually between them. Some women use those words as well. 년 means 'bitch' in this context, you wouldn't want to use this word.