Can I add my own models to the game, either to replace existing models or to augment the game?

If so, is there a tool to convert them from popular formats?

I'm sure you would be able to at some point, as long as they don't resemble Keanu Reeves or other random celebrities which object to putting their virtual three-dimensional alter ego in all sorts of situations and positions :)

check nexsusmods, i've seen there mods that added new posters to game or changed appearance of adverts on street.

as i've understood there is cp2077 tools that allow to extract data from game archives and to repack it back, tho for more complete info you'd better ask on discord server which is dedicated to cp2077 modding

Right now, there's model swapping. It works really well. To add one's own models is only a matter of time. There are brilliant, creative and intuitive people working on this game. The modders have already taken it from a wonderful to look at game to an tolerable playing game. In time, it will become something uniquely wonderful. IMHO

Once more refined tools are available or created, there will be an explosion of additions, both good and bad, to add to your gaming experience.

modders made it available to add own models to game
check this out :)
https://cdn.discordapp.com/attachments/716476859382235186/805761173583822848/Cyberpunk_2077_C_2020_by_CD_Projekt_RED_2021-02-01_12-27-12.mp4