In order to have a wireless system in a space, you have to buy a wireless reciever, headsets, etc. When my college did this it cost around $15,000 in 2001. The only real requirement to combining wired and wireless is that they must be the same manufactuer, mainly Clear Com or Telexx. I've used both and haven't noticed a difference.
There is a company that sells a system where for each of their wired headsets, you can run 2 wireless. The wired packs plug into you existing ststem and you can get started for about $100. I forget who that company is at the moment, but I think there is a link on this site.
Some people use walkie-talkies as an inexpensive and transportable alternative to intercom systems. They are much cheeper and you can get a headset attachment to avoid the whole theater hearing all communications. Personally, I don't like this option. Only one person can be talking at a time, so no one can respond to you until you stop pressing the talk button. Also, they pick up a lot of other signals- like those walkie-talkie cell phones that are so popular.