It is neither active nor passive voice. It isn't a correct phrase at all. It most closely resembles a command but for that it should be 'Welcome your friend'. Or it could be a statement - 'I welcome you, friend' (although this one doesn't sound very natural). Anyway, it can't be changed from active voice to passive or vice versa. Is it possible you have misremembered it? If not, I think it's a bad test as the language in it is not correct.

"Welcome your friend" (imperative) can be changed into "Make your friend welcome", which is not passive but by changing "welcome" to "welcomed" it becomes passive. But really, it sounds better to leave it as it is: "Welcome your friend".