I would have to concur with Larry. Most mortgages are not assumable. Is it legal to make payments for someone else's debts? It is not illegal but my question is why do you want to do that? The same question that Mark asked.
Try to get a loan and buy it from the owner. Caveat: Make sure you are paying for what it is worth for.
If you really like the house, seek the service of a realtor to guide you in the process.
Very few mortgages are assumable and while no one can stop you from making payments on a mortgage, unless the mortgage clearly has a clause in it allowing assumption (trust me the chances of this being the case are slim to none unless it's a VA mortgage and your a veteran) you're wasting not only your time but your money.
You should contact a local attorney to have them review the mortgage documents before doing anything.