Of course he can, but that's not how it works. You don't just transfer that kind of political power like it's a costume. Transfer of power of this magnitude is a very big deal.
T'Challa not being king when he came back was easily explained on two occasions (Doomwar and FF): he didn't want to wrestle back the throne from Shuri. He chose not to do it.
It more or less worked out in the end, as Shuri transferred the crown back to him before her death in TRO.