Hi,
Yes its possible to migrate from aspmembership to mojoportal membership, you would need to map data from the aspnet membershhip provider tables into the mojoportal tables mp_Users and you may also need to migrate role memberships into mp_Roles and mp_UserRoles, it could be done by writing queries to migrate the data.
I don't know about the YAF stuff, other than to say that someone in the community has got YAF integration working before, there is a link to it on this page http://www.mojoportal.com/community-custom-features.aspx
I do not know if it still works or what the status of it is or how to use it.
Hope it helps,
Joe