Combine Control (the thing TnB's whole gamemode based off of) has its own weapon base, I doubt any other weapon base could even be transferred to it even if someone wanted to. If TFA were to even be 'added' it would be in models only, all those other little gibs that are in the TFA system would have to be put into the TnB weapon code, which would be pretty hard considering its not been done.
Combine Control is, in my personal opinion limiting, but a much better base then any other things out there like Clockwork or Nutscript. Its hard to get game changing addons in, though i'm sure the devs could do it, Tanknut is a miracle worker.
In theory we could port TFA into CC/Eternity, there just isn't really a point in doing so. As mentioned before, we have our own base and we've been continuously iterating upon that base to suit our needs.
TFA's base has an absurd amount of configuration options, a lot of which have very little impact or would take ages to 'learn' how to get right (Take a look at this to get an idea of how much goes into a TFA weapon, compared to our own base which asks for a lot less in comparison) and good luck convincing Dave to learn all of that.
We can edit our base at any point, using something like TFA would force us to rely on them in the event of any issues or have to try and understand their code and fix it ourselves.
There's a lot of stuff included with TFA that we don't need or we handle in other ways.