Like 1sourdough, I don't recall ever seeing any mod kits or hi-cap mags for the BAR sporting rifles.
However, if you're looking for a brand-new rifle, you might be interested in the recently released FN FNAR-- a "tactical" version of the BAR in .308 with a detachable box magazine and a pistol-gripped stock.
Also, there is a company in Arizona called Armstech that makes (or made) a "sniper" BAR, apparently in limited use by US special forces at some point, chambered in .300 Win mag with a 10-round magazine. There is a GBO thread about this rifle here:
http://www.gboreloaded.com/forums/index.php?topic=131625.0