That's only through breeding though. as far as the buggy transformation menu is concerned, imp is not an accessible race.
(Edit: I could have sworn there was a line of dialog during the story that said humans were discriminated against because all sexual encounters led to imps)
this is actually not true. You CAN become an imp through the buggy transformation menu, its all about how you change yourself in order due to the weight system of determining your race. The first thing you HAVE to do is set yourself to at least below 4 feet tall (imp have a height limit determined for demon subrace), only after you're a short stack then you change all your body parts (except genitals, they do not count towards race weighting) to demonic and demonic only, no exceptions. Then you'll be recognized as an imp. Interestingly, when you become recognized as an imp you can then scale your height back up past the height limit and still be recognized as a imp, its only when you save with that extra height and reload do you become recognized as an incubus. This does not work the other way around. You cannot be an incubus, shrink, save and reload as an imp. So you have to keep all this in mind when trying to become an imp through the buggy transformation menu.
Or you know... just use the race reset option.
sidenotes: 1. this was tested in the 3.13 version, i assume this works well enough the same in 3.15, but with the recent focus on race modding and stuff you never know, so im just pointing this out.
2. after reloading as an incubus i used the buggy transformation to shrink below 4 feet, saved, reloaded, then tried to transform (using the legit demon transformation menu) back up and interestingly enough i cant get past the height limit just as a regular imp cant grow out of their height limit. So something in the game does see me as an imp in some way because functionally i cannot grow out of their height range. so I fixed this by just using the buggy transformation menu, grew past the limit, saved and reloaded, tried to shrink down past the limit and couldnt. This is all likely to keep the player from becoming a demon, shrinking down and the game weighting them as an imp when that goes against the lore, but the buggy menu transformation kept me as an incubus even after going down past that limit and reloading the save with the height... so... idk. food for thought I guess. This height transformation restriction applies to all races who can transform, so that includes slimes and youko.