I'm sorry, Maruem as an Emitter when his power was specifically to eat brain matter and gain nen powers from the prey is simply ridiculous. That makes zero sense to me whatsoever. The otehrs, like Chapter 7 Bankruptcy, are interesting and I actually assumed it was Conjuration due to how it worked a lot like the exorcists nen beast. But that is beside the point. It seems utterly bizarre Maruem is like he is.
Of course, you can become a Specialist. Machi is a specialist, but her original category was Transmutation. That seems to be what I'm seeing from this. I guess Maruem was always naturally Emission but his power as Ant king was to absorb new powers and so he is kind of a specialist / Emitter. Which would explain the absurd power boost in the abilities he absorbed, maybe. Since they were all emission based. (The explosive beam attack and Spiritual Message were technically ranged).