If you manage to get a Matrix/Slitherine key with your battlefront key, a steam serial appears at your Matrix/Slitherine page.
With this key you can add your game to steam here: https://store.steampowered.com/account/registerkey
(as far as I know the Matrix-Steam link doesn't register your M/S games at steam automatically)
I am just a bit lazy, I have the battlefront version, bougt the 1.0 version (many years ago), and all of the upgrades and packs. reinstalling is a bit of a mission on its own though, also steam usually keeps things up-to-date for me.