Unfortunately that is impossible.
March to October rosters are fixed and can't be altered in any way prior starting a campaign. I don't think they'll allow it in the future since there are rewards in that mode.
But you're Right, it's the only playable mode... Since launch I've played 5 campaigns of MTO and maybe 20 games of RTTS... It's sad since I used to be an almost RTTS only player... I try to get my money's worth for that game and keep RTTS with TheShow20, Which was 1000 times better...