I want to add to my seed ratio (on private tracker with UL Ratio Rules) without downloading more files on a huge (1000+ file in a single directory) partially downloaded torrent.

Can I seed the stuff I have without turning off unfinished files one by one, or having to download the remaning blocks of the entire torrent?

In order to do that, set all files inside that torrent to OFF (select > right click > Priority > Off). The torrent will show 0% completion but it will seed already downloaded pieces just fine.

Can I select multiple files and do them all at the same time? (If not can it be added as a feature?)

It would take me a good hour or so to do them one by one.

Yes, ctrl+a will select all, shift-select also works, then right click > Priority > Off.