Got a search for that, and it occurred to me that it is a valid question. The answer is yes.
The degree varies. You can simply contact the bank to make yourself responsible for payment. They are usually happy to do this, although unlike revolving accounts you typically will not receive back credit on your credit score for the entire length of time the trade line has been open. Nonetheless, if the bank reports the mortgage as paid as part of your credit, it can help you increase your credit score, so long as the mortgage actually gets paid on time every month. One 30 day late is plenty to kill any advantage for most folks. This is typically free. Hey, the bank has one more person to pay the mortgage! This is often used as a way to start rebuilding credit after a bankruptcy or other financial disaster. A friend or family member qualifies for the loan, then adds the person looking to recover to the loan later.
If you want to go one better than that, you can actually modify the deed of trust to make yourself responsible for payment, although it really has no measurable benefit as opposed to simply agreeing to be responsible, and it costs money to notarize and record the modification.
Unless you can get a better rate by doing so, I would advise against a full re-qualification for the mortgage just to add someone. It’s a lot of hassle and expense for no particular gain. If you want to get me paid, I’m cool with that, but there are better ways to accomplish the gain to your credit at far less expense.
Note that this is a different proposition entirely than removing someone from a mortgage or swapping out one person for another. They will NEVER remove anyone from a mortgage without a full refinance.
Caveat Emptor