You can use the Python library fontTools, which comes with a command line tool called `fonttools varLib`. Prerequisites: 1. you have a so called Designspace file which tells the tool where in the design space the fonts are and 2. The fonts have compatible glyph outlines and font features.
For the former, get a random font project like https://gitlab.gnome.org/GNOME/cantarell-fonts/-/blob/master..., install the Python tool fontmake in a venv, and run `fontmake -m Cantarell.desigbspace -o ttf-interpolatable`. In a new subdirectory, you'll find a new Designspace file and TTFs. Switch them out and edit the DS file to make sense. Good luck.
For the latter, you are probably out of luck because static fonts are usually compiled in an outline-incompatible way and your only options are to fix them yourself in fontforge or such or ask the original vendor, who is probably going to laugh at you and demand money.
For the former, get a random font project like https://gitlab.gnome.org/GNOME/cantarell-fonts/-/blob/master..., install the Python tool fontmake in a venv, and run `fontmake -m Cantarell.desigbspace -o ttf-interpolatable`. In a new subdirectory, you'll find a new Designspace file and TTFs. Switch them out and edit the DS file to make sense. Good luck.
For the latter, you are probably out of luck because static fonts are usually compiled in an outline-incompatible way and your only options are to fix them yourself in fontforge or such or ask the original vendor, who is probably going to laugh at you and demand money.