In theory - and without having looked at it in detail - this might be possible. We wouldn’t need GUI changes even. The user sets the framerate with the existing BSpinner and off we go.
The one tricky thing is how to select the source images. Easiest, but not super user friendly, would be to make the user put’em all in a folder and have it selected just like you select a video file. The app would check if the selected folder only holds images and goes into image2video mode. Relatively easy, but not intuitive, people would need to read the docs to find out how it’s done.
Better would be to have the user just select all images. Depends how those are fed to ffmpeg…
Anyway, it seems possible without much/any additions to the GUI. Though, the devil usually lurks in the details… 