There's always plenty of talk about booking a 2-4-1 outbound flight and then adding the return later, but not so much the other way round.

I have a new style UK Amex 2-4-1 voucher. I want to do an open jaw booking which obviously requires calling. There is availability for my outbound but the return is from a more popular destination and I know I will have to book at midnight at T-355 to secure it. Can I do that online (or at least, try to!) and then call the following day to add the outbound (paying £35 p/p I know as the outbound is already available)?