Virgin flying club - Add new flight to existing itinerary?

The good folks at V Flyer have been very helpful with my daft questions so far, but I can't get a response on this one and time is of the essence!

I was wondering if it's possible to add connecting flights to an existing VS itinerary (VS to BD) and if so, how? Is the FC helpline the number to call? Should I buy the flight first then try to add it or can FC book it for me?

Or can it just not be done at all? The existing booking is a one-way UC Miles and paid-for PE.

Many thanks!

IME (and not with VS!) adding flights to an existing PNR can be difficult to impossible.

I think I'd call FC and see what they say about it - before I booked the flight elsewhere.