Basically a mortgage is a secured loan, the condo serves as collateral. In the event of default by your Thai wife the bank has the right to seize the condo and auction it off to settle the outstanding balance of the loan. The bank will not allow you (your wife) to register a usufruct over the condo after the loan and mortgage registration (read terms and conditions with the bank) as in the event of default on the terms of the loan they would not be able to sell a condo for a reasonable price because of the registered usufruct. Who would buy a condo that is burdened with a usufruct? The whole security for the bank would be lost if there would be a usufruct registered after the registration of the mortgage. So, no this usufruct would not be possible.