The standard RA1160 fixed base for the Bosch 1617 router can indeed be converted to the RA1165 undertable mount configuration. Be advised that it can be done only on the aluminum bases, not the magnesium bases. The aluminum bases have the letters "AL" cast in the housing just below the clamping lever. Magnesium bases have one of the baseplate attach screws located right where the hole needs to be drilled. When you remove the baseplate of the RA1160 base you will see that Bosch has already partially drilled the location for the hole needed for the longer "adjusting rod", (Bosch P/N 2 610 922 527).
All you will need to do is to drill the rest of the way through the base, install that part, and drill a matching hole in your router table's insert plate. You will also need a replacement knob (Bosch P/N 2 610 993 576) as the knob on the original adjusting rod cannot be removed without destroying it. You will however be able to reuse the original "indicator" sleeve (Bosch P/N 2 610 993 577). You will also need a 1/8" allen wrench for making bit height changes. The undertable mount configuration really simplifies making bit height changes. One turn of the wrench equals 1/16" and one quarter turn is 1/64". I've found that accurate height changes can best be made by gradually raising the bit instead of lowering it. Also be sure mark your insert plate with an "UP" arrow indicator. Once you get used the setup, you will find that you will rarely need to bend down to view the indicator. By feel, you can loosen the clamp lever, make the adjustment from above the table and re-clamp the lever. Again, since the location of the clamp lever and the router switch and speed control always remain in the same place, most of the changes under the table can be done by feel.
PS - - If you have trouble getting the adjusting rod from Bosch, try Sears. Their Craftsman Router, # 130.26620 is nearly identical and has the same part numbers for both the adjusting rod and the knob.