Without all the technical info of what you hooking up and what the existing load,If it meets the tap requirements of article 240 and not overload the original load, and meets other applicable requirements, I don't see a problem
I have a problem with it. The whole idea of paralleled conductors is that you are going to balance the load across the conductors equally (same size, length etc) If you unequally load one conductor, you are defeating the O/C protection. The only way I see this as legal is if you group them, make your tap and then break them out again.
Where individual conductors are tapped from conductors in parallel, the tap connection must include all the conductors in parallel for that particular phase. Tapping into only one of the parallel conductor would result in unbalanced distribution of tap load current between parallel conductors, resulting in one of the conductors carrying more than its share of the load, which could cause overheating and conductor insulation failure. For example, if a 250-kcmil conductor is tapped from a set of two 500-kcmil conductors in parallel, the splicing device must include both 500-kcmil conductors and the single 250-kcmil tap conductor.