Most mortgages have a "due on sale" clause that the lender can use to require the loan to paid off, if there is an attempt to dilute or reduce the owner/borrower's interest w/o lender approval. You probable just need to talk to the lender and explain what is going on. Ask if they will allow you to be added to the deed. They may require you to become liable for the loan before allowing this. It's all about protecting their interest.

If this doesn't work, then a "contract for deed" could be used to initiate your claim to partial ownership. This wouldn't get your name on the deed anytime soon, but eventually it would happen. If you go this route, you should consult an attorney.