Yes.
When in therapy we frequently had new ones come up. meaning... as we processed different aspects of life / memories, we began to work with the alters that were triggered up. They were always there of course. Some of them may even have been active in recent times. By 'new' we meant they were new to our awareness.
We still have a ton of 'new' ones to process. They are the ones we are still amnesiac for... the ones that still hold dissociated experiences and information for us. We don't know who they are, we have never met them before. And when we do... we will most likely say to our t, to our friend, to this board, whatever... "we had a new alter come up today."