Adding dimensions to an existing TF-IDF vector

[Mark]

We are trying to cluster users together by the type of products they 
sell. For this we are building TF-IDF vectors on all of the products 
each user sells. From these vectors we create our clusters and our 
initial results aren't too bad. We would like of course to add some more 
information along side the TF-IDF vectors.. perhaps the categories that 
each user typically sells in.

Is it possible to add more dimensions to an existing TF-IDF vector?  If 
so how would it be possible to determine what appropriate weighting to 
give to these new fields to make sure its not too much/too little?

Thanks for any input