I was surfing around about news regarding the new season of In Treatment, on whether Melissa would be scheduled to appear in it, and I came across this article (I will only show you a part of it)

“When the show does return, though, it will do so with some pretty significant changes. Paul will likely have a new roster of patients visiting his home office. HBO’s press release says only that “additional cast members will be announced as they are confirmed,” which implies that first-season patients Melissa George, Blair Underwood, Mia Wasikowska, Josh Charles and Embeth Davidtz may not be back.” Source.

Of course there’s nothing official but juding from the other things Melissa has going on, I would, personally (know nothing for certain), not see it likely that she will appear on the show more than possibly a cameo appearance.