I don't have a specific answer to your question, but I will say that we converted from a somewhat (to me) obscure document management system, and we had to pay our LF provider an hourly rate for professional services to do the conversion. It was done via multiple workflows and a couple extra VM servers. It was painful at times to get done and took literally months for the conversion to fully run, but it got done. I and the person designing the conversion workflows had to research database structures and links between tables to determine how things tied together. It may come down to talking to your LF provider and paying for professional services to get it done.

EDIT: And after I looked I saw you are a VAR, so you probably already know everything I typed, but I'll leave it in case it might help others in the future.