Re: TRG mags ?
I think the factory mag is only 6 or 8 rounds. Getting a 10 rounder would require having two mags welded together and finding an appropriate follower spring. It could be done, but the expense would be pretty extreme, and simply inserting another mag might be a cheaper idea.
With the distances at which the 338 is normally used, I think there should be ample time for a mag change.
But to directly answer your question, NO, I don't think there is either a factory mag, or conversion to get 338 Lapua mags for the TRG42.