I recommend to check our terminology, but this question is also a bit unclear. Am I right in assuming you mean people who identify as individuals coming from alternate universes? In that case, there are several options. If the alternate universe is described in any fictional source and the person in question would identify as a character in that source, they'd be obviously considered fictionkin. If that person would identify as a character who doesn't appear in the source, I'm not too sure but I guess the term fictionkin might still apply because one could always construct an alternative storyline.
If, on the other side, the alternate universe would be totally unknown, the term "otherkin" would imo only apply if the person would "either partially or wholly as one or more non-human beings, entities, or concepts" (cp. terminology). In any other case, none of the terms would apply.
Finally, there could also be fictionkin in a self-designed source.. I think I didn't see this yet, though. But I wouldn't be at all surprised if authors would be describing their own life story in an alternate universe. Could be a way how fiction comes about in the first place if you think about it.