I live close to the East London line and was wondering if the ticket staff at these stations can (and are allowed to) issue advance purchase tickets for other TOCs?

I've never seen anyone use the ticket offices in any of the former LU ELL stations so it could be an easy way to buy an advance purchase ticket when I need to use up some rail travel vouchers.

Normally when I'm using vouchers I have to queue at those awful 'tickets for future travel' desks at a mainline station where the queue moves so slowly because they're searching for the cheapest fare for people, whereas I've done my research beforehand and know exactly which trains I want to travel on.

If London Overground can't do this, what about the ticket offices in smaller mainline stations?