Workflow Task: Limit list of users in the drop down for a WTUser variable

Hello Gurus,

I hope you can help me.

I have a requirement to show a variable (of type WTUser) with a drop down in the workflow task.

I have create a workflow variable of type wt.org.WTUser and made it visible, which is rendered with a drop down.

However, the drop down lists all the users in the LDAP.

How can I limit it to show only few users programmatically (e.g. the users in the role "Design Engineer" for the team instance)?

Any idea what to customize?

Thanks

Antonio