Really no way to add another tuner to a single tuner TiVo. Your life time subscription is tied to the mother board not the hard drive so moving the hard drive to another box will not move the lifetime service.
The best you can do is hope for another opportunity to move your lifetime service to a new box. TiVo has had 2 such offers in the last 12 months, one open to anyone buying a new Series 3 TiVo and pay $199 to move the lifetime service and a second time to select accounts to buy a series 2 DT unit and again pay a $199 fee to move the lifetime service .
Good Luck,
The best you can do is hope for another opportunity to move your lifetime service to a new box. TiVo has had 2 such offers in the last 12 months, one open to anyone buying a new Series 3 TiVo and pay $199 to move the lifetime service and a second time to select accounts to buy a series 2 DT unit and again pay a $199 fee to move the lifetime service .
Good Luck,