Yes, this is fine. Just present the ticket and ask for a Plusbus at the ticket office.
And do bear in mind that they are only supposed to issue ones for the origin and destination station printed on your ticket.
So if you are breaking your journey, or have bought a ticket for a station further away than where you plan to get off, or have a ranger/rover/geographical rather than point-to-point ticket, they may not sell you a PlusBus ticket for the station you wish.
(On the other hand, you might run into someone who interprets the rules a bit more laxly. For example, I've managed to get a PlusBus ticket for a BritRail pass, which has no station names associated with it, on stating that on a particular day, I would be using it for travel from A to B.)