Assign URL parameter to User ID

Hi, I’m tracking email campaigns with url parameters (pk_campaign, etc) and i want to assign one parameter (pk_content) to User ID in order to track individual visitor log.
I think that this should be done somehow using setUserId method. I don’t want to use any userid from login to my website (i have already seen from some examples), i just want to use url parameters.
Is it possible? Is there any other suggestion?