The practicality of connecting an implant to a natural tooth using a bridge is the subject of much controversy in dentistry. There has not been any substantial evidence to support or refute this dental procedure. The majority of dentists do not suggest connecting an implant to a natural tooth. In any case, nothing should be attached to an implant that places it under any functional stress until the bone has grown into the implant surface. Bone attachement stabilizes the implant and takes about three months.
The natural tooth is supported in it's socket by the periodontal ligament. This ligament acts as a shock absorber for the tooth when it is put under compression during clenching, chewing, etc. It provides a small degree of tooth mobility so that the tooth does not fracture.
The bone which grows into the implant surface results in a rigid body in the jaw. There is no give in this system. Stresses are prevented by carefully adjusting the occlusion so that the implant restoration receives minimal occlusal forces.
When a bridge is used to attach a tooth to an implant, the tooth end of the bridge retains the inherent mobility of the periodontal ligament while the implant side is fixed and rigid. This difference can result in the bridge retainer attached to the tooth separating, the implant failing due to loss of bone attachment from the motion applied to the bridge or the tooth or implant body being intruded or extruded from the jaw resulting in an unbalanced bridge and potential occlusal interferences.
Personally, I prefer not to risk the longevity of the bridge by using like abutments (tooth/tooth or implant/implant). You should discuss the long term survivability of a tooth to implant bridge with both your surgeon and restorative dentist.