How to get interdisciplinary when your program isn't
You need to pick an advisor who will support your interest in going outside of the area of his own work.  More likely than not, that means somebody who you both get along with well and who is tenured, so that they are not feeling pressure to "get work out of you" towards their own career goals.  Be up front about wanting to expand your area of interests.  You may, in fact, find a professor who is interested in supporting you in this, because they are interested in expanding their own interests, and the best way for two professor to begin collaborating is often to be co-advising a student who stands between them.  

Which brings me to my next point: you are likely going to need to start connecting with professors in the other department where you are interested in working as well.  Start sitting in on seminars, get to know people, and see where you can find a connection.  You might or might not end up needing formal co-advising, but you will certainly need to have at least informal mentors who you can go to for advice in the other discipline.

What is certain, however, is that you need to be very proactive about searching out your interests and taking on risky projects that will let you "get your feet wet" and figure out how you relate to the other field.  You are also likely to take longer to graduate.  Interdisciplinary research is a much harder path, but also potentially a much more rewarding one.