[sli]

This is usually how it's done. See, for example, OpenMW. It extracts the resources from your already-installed copy of Morrowind during the installation.

E: Also, some total conversion mods will make a copy of your game installation, then install itself into that copy. I think (it's been a while since I've played them) that Nehrim (Oblivion) and Enderal (Skyrim), both from SureAI, do this.

[cyphar]

(Though OpenMW isn't a mod. It's a reimplementation of the engine.)

Bathesda also had some very strange requirements for the OpenMW folks when they were in contact[1]. Namely they didn't want them to show OpenMW working on Android (because Bathesda wanted to port it themselves?) nor did they want them to support the XBox version of the game. I doubt that Bathesda actually had the right to add those requirements to a project that is a from-scratch reimplementation of their game engine, but that's the current status.

[1]: https://wiki.openmw.org/index.php?title=Bethesda_Emails