Riot's Launcher is pretty bad when it comes to this and the company hates accessibility, so it's not coming to Steam. The only way to add 2XKO or Valorant as a non-Steam game is to add RiotClientServices.exe as the game, then set the launch parameters. Unfortunately, this does show my Steam status as playing the non-Steam game, but the overlay doesn't work.

This is how my 2XKO Steam properties look:

Target:
"D:\Riot Games\Riot Client\RiotClientServices.exe"

Start In:
D:\Riot Games\Riot Client\

Launch Options:
--launch-product=lion --launch-patchline=live

"Enable the Steam Overlay while in-game" is enabled.