I was able to add a sector to a recent booking with little pain via the ICC:

AKL-SYD-BKK-MUC-MAN (all F, except AKL-SYD in J)

became

WEL-AKL-SYD-MUC-MAN (Additional section in Y as no J award space)

for the change fee and a very little more tax.

So, just give them a ring and ask them to tag on the additional sector.