Yes, you need a model of the interior (already in game), the XYZ co-ordinates, and a feature called an ENEX (entrance/exit),
the bouncing yellow arrow.
Look at MAP mods in the download section, & get a feel for how they work. They are IPL edits so notepad can open them.
Look for mods by Superdario. I've seen some of these w/ ENEXs.
ENEX can also be placed w/ scriptmods. There's one called "Hidden Interiors", I think it includes Ryder's house, but warns of
messing w/ missions.
Good Luck.
EDIT: BTW, welcome. If you haven't already, look over the forum rules & the general layout.