Currently there is no way to adjust the O2 Supply. The eventual plan to have a customized DLL for each of the major models of Capsules. The current version is based off of Grissom, Glenn, and Carpenters capsule.
I'd be interested in this whenever you finally get back to it, aside from MA-09, there is the unflown MA-10 (three days) and a fictional four day mission I'd love to test out.