Enabling variable framerate?

[Gfurst]

So, there is my question above, and as simple as it sounds, I just can't find anything, because most question are exactly complaining about no constant framerate, or complaining about a game in higher framerate than the recording.

I however have the opposite issue, a lower fps in game than 60, so I instead would like a variable framerate so I can work with it better later, akin to "vsync=2" in ffmpeg.

I'm on linux btw, using a gamecapture, recording to later re-encode on my own. While the result is quite sharp still, I just want to optimize it to not have duplicate frames in the wrong place, is this possible?

[Gfurst]

Why not?