Sure you can do a lease-to-own. Lots of lease-options and lease-purchases are done on properties on which there's a mortgage.
One potential risk (which a couple of folks referred to) is the "due on sale" clause. That's a clause in the seller's mortgage that says that if the owner sells or transfers an equitable interest in the property without the lender's approval (and the lender won't approve), then the lender can immediately call the loan. Most loans--except for the very few that are assumable--have a "due on sale" clause.
As a practical matter, it's unlikely that the lender will foreclose on a property where the payments are current and coming in on time . . . though, technically, it could happen. There are a number of strategies to reduce that risk; a Realtor familiar with lease-options can advise you on them.
Hope that helps.