I am not a modder but you can turn them from a minor faction to a major one by going into the totalwar directory then go into campmap -> startpros -> early. And then where it says SetActiveFaction: FN_NOVGOROD FT_MINOR change it to FT_MAJOR. I think you'll also have to change all mentions of Russians in the text file to Novgorod as well. Make backups just in case.
I think that's how you do it but like I say I am not a modder but I know bits and pieces.