Step 1. Buy a 3-band preamp.
Step 2. Remove old 2-band preamp.
Step 3. Install 3-band preamp.
I'm a fan of the Aguilar OBP-3, and it's probably the most reasonably priced for the amount of boost/cut power that it has. However, John East, ACG, Nordstrand, and Audere all get fantastic reviews as well. In another thread, I did post some of the technical specs for some of the major pres and frequencies that each band affected:
Wick Taranis vs Aguilar OPB-3
I couldn't find exact information on the John East or Audere, but I'm sure it's out there.