Say a character takes a weapon class they would be able to gain Martial Weapon Proficiency (Longsword) as a bonus feat.
I'm not understanding this request. If you take a class that grants an additional feat, then the class should be programmed to do that. If you're asking to be allowed to gain additional feats at a whim, then simply alter the number in the available feats and change the number as desired, if you want to track a GM bonus granted feat, then use the GM Award section and grant the number of additional feats allowed, if you want to bypass the requirements of any given feat, then simply turn on the bypass feat prerequisite rule in the preferences.
If that doesn't suffice, then please explain in detail what you are asking the Data team to add and cite why you would need it (Homebrew is covered from every angle here, so I cannot imagine another scenario that would require some other add a feat mechanism).
I did not realize you could simply change the number under "Remaining" to grant additional feats and the documentation makes no mention of this, you may wish to make it more obvious in some way.
GM Awards allow for additional feats, as does direct value manipulation. Closing.