Can I have a PhD in one field, and be an avid academic contributor in another?
In principle there's no problem with that. Many researchers change their fields over the course of their career -- sometimes slightly, sometimes drastically. No one cares what your PhD says on it, just the quality of your work. 

There are a couple of caveats. If you need funding for the new field, you may have trouble convincing funding agencies that you're competent to perform the work.  And of course it can be hard enough to do good work even when you devote all your time to one field, let alone two (though on the flip side, using insights from Field A may give you a new view on Field B). 

But the field your PhD is in, by itself, doesn't limit your activities.