I really wouldn't mind at all - it makes sense to me in character, as we've lost members of the unit earlier in missions, so someone new being transferred in would shore up the gaps. And the unit is unconventional anyway, given we're from all different nations, and dealing with rapidly changing situations, so I could see the actual organization and supply of it being a bit of a hodge-podge in kind of an eternal 'fix-up', which would explain things away. 
Out of character, I think a new member wouldn't be a bad thing. Especially since Zenku's been consistently hanging around and eager to join in, rather than just dropping by once and asking to play.