TromboneChat

Can you modify a single valve bass trombone into a double valve bass trombone?

The answer is "yes, but".

A conventional wrap Bach or Holton can have a valve that plugs into the narrow loop of the F-attachment. These aren't common.

Any other bass (open wrap, or a wrap like a King or Yamaha) will need some serious surgery to add another valve and appropriate tubing. This modification will cost around $2000 (US). Not a casual thing to consider. It may be more cost effective to find a used 2 valve bass.

Some more information would be helpful.
What horn do you have? Are you wanting a removable second valve? Or are you wanting to convert your single into a permanent double valve setup?

It is a Bach 50BO that I want to convert to a permanent double valve

Physically possible, but quite expensive. The most straightforward way is to buy a double valve section (from Shires, M&W, Instrument Innovations, or someone else) and replace your existing single valve section. The parts (if purchased new from manufacturer) could easily run a couple grand, and the skilled labor of a competent tech to install it properly is not cheap either. Buying a trombone that already has two valves is likely the path of less resistance.

Okay. The question makes sense now.
What type of valves are you wanting?

As an example, I (M&W Custom Trombones) offer a double valve section, complete, using my own design M&W rotor valve, installed on your bell/tuning slide, for $4,275.00.

Here is a picture of one such project....