Variable upload paths possible?

Hello,
I want to use FCKeditor on my multi-hosting content management system. It will define what site it is actually working with and make a session variable stating what sitenumber (for example sitenumber = 4 for this example site) and then all image or file uploads should go to /images/4/ and so on.

Problem is, how do I make the upload destination folder variable like this?

I am using cfml.