I need a workflow the same as the OOTB approval workflow, except the approvers list comes from a field in the current item.
Is there a way (in SharePoint 2007) to edit the existing workflow to allow this? Or do we need to create a workflow from scratch?
You need to create a workflow from scratch. If you upgrade to SharePoint 2010 you can create a new workflow from the Approval type and modify it to fit your needs however.
If you are planning a 2010 upgrade any time soon you would save yourself some development time going this route.
