Rimworld on steam and mods from this forum

[TrueDestroyer]

Can users of steam version install mods acquired outside steam workshop?

[Facepunch]

Yes

[DanielCoffey]

When you unpack a forum mod, it will be in a folder. Unless the mod author says otherwise, just drop the entire folder into the Steam / Steamapps / Common / Rimworld / Mods folder and restart the game.

The nice thing about the in-game mods list is that it has a different icon for Steam mods and ones that are manually dropped into the Mods folder. Nice touch!