Update avaialble step assignees based on data element in upper form

Hi,

I am trying to get the list of possible step assignees within a specific step in a work flow to be filtered on both the user group and a specific data element in the upper form of the BP record. I am using a Simple BP type and in the data elements a user has imported the line managers name. Assuming the value of the field is the username, is it possible to then filter the possible assignees based on this field?

Thanks for your help.

Dave