1. why not use the other columns as evidences and come up with a preference score <UID> <ITEMID> <PREF_SCORE> and see if results improve.. May be you use other machine learning algorithms to generate such preference scores...
2. one other way may be to implement a custom similarity Score and not the ones that ships with mahout where you can use this column values to decide on the similarity of the users. Kindly have a look at mahout in action. There is an example for dating recommendation. This problem of yours <USERID,ITEMID> can mapped back to the same problem mentioned. try implementing the similarity score using the other column values. May be some expert in this area can come up with a better solution...if i were you i would certainly test the waters like the way i mentioned... Parnab... CSE, IIT Kharagur BIS, University College Cork
