I researched about this topic and I agree that there is no default support for a USB 3G modem but that is just poor use perfectly capable hardware by linksys.
I'll try to get an anwser from the folks over at dd-wrt as you suggested but I am not willong to buy a new router no matter how cheap it is(I despise myself for buying E4200! Could have gotten myself a decent netgear and even saved a few bucks in the process!).
Thank you for your time.