One way to do it is populate a dummy attribute with the account manager name and then have a workflow trigger off the creation of the lead and assign appropriately.
Leon Tribe
Want to hear me talk about all things CRM? Check out my blog
http://leontribe.blogspot.com/
or hear me tweet @leontribe
Want to hear me talk about all things CRM? Check out my blog
http://leontribe.blogspot.com/ or hear me tweet @leontribe