Up until a couple of years ago, you could book Manchester - London - New York with MAN-LON being in June (before your voucher expires) and then a long stopover until the LON-NYC leg. This is no longer possible, and I'm pretty sure what you want to do can't be done either.

You can easily find out, though, by doing a dummy booking and trying to move it. No charges for changes made online and you can then cancel and get the voucher back.