I'm working on a project that's texture-heavy, and I'd like to skip long re-imports by converting my textures into whatever it is that Unity is going to compress them into anyway.
I'm hoping this also will stop Unity from having out-of-memory crashes when I build.
What format does Unity compress its textures into?
If you use .dds format for the textures, I don't think they'll be recompressed.
Unity most likely compresses everything into asset packages using a proprietary format, and you wouldn't be able to convert your textures to that.
How big are your texture files? Are you using source files (like .psd or layered .png)? Are the dimensions really large? Most texture files shouldn't be over a few megabytes depending on the quality involved, and shouldn't be over 2048x2048 (even that's pushing it).
There comes a point where your textures are too detailed and too large to even be noticed in-game (after ansino levels and/or mipmapping are applied). If you're asking this question (how can I do what Unity already does), then you may need to size down your textures a bit.