i believe you can switch the add-on domain to be the primary domain, create the subdomain as normal, then set the original domain back to being the primary domain.
at least, i've managed to do that to add a subdomain to an aliasdomain, i've not done it with an actual add-on domain, but i see no reason why it would behave any differently.
but yes, it would be quicker, simpler and much more elegant to be able to just fill in the subdomain field and be able to use a drop-down list to select any of the existing domains in that subscription/package.
possibly even also have a checkbox option to apply the new subdomain to every existing domain in the package in one go.
might not be needed very often, but would certainly save quite a bit of time and faffing around when it is needed.