Hi Jeffrey,
I've been working on similar...
You'll want to explore QuerySpec and Code examples for generating a Query within an Expression Robot. This will find you a WTPrincipal based on Name (username) or other property such as Full Name. From there explore RoleHolder (public API equivalent ofr TEAM) where you can add the WTPrincipal to a WTRoleHolder (Team) of the pbo. Note - your pbo Team will have to have the Role in question, and I'm assuming that the WTDocument Lifecycle is Advanced with Workflow.