You can do this by modifying the fixture profile. You will need to change the profile to specify that. say fixture 1, has as many channels as you would like to compile into a single fixture.
For example, if you have 3 fixtures that are 4 DMX channels each, and you want to control them as "fixture 1", you will want the profile to reflect that fixture 1 is a 12-channel fixture.
This is accomplished in the fixture setup/patching in the menu. As a result. when fixture 1 is selected, channels 1-4 control the first unit, channels 5-8 control the second unit, and channels 9-12 control the third
You have to be careful to make sure all of the other settings reflect what you want to pull off. For example, you'll have to make the blackout channel "ALL" so that all three units will black out when fixture 1 is selected. Unfortunately, this will cause gobo, color, pan, and tilt information to be lost because it will turn "all" channels to zero, rather than just the shutter or dimmer channel. This is one of the downsides to operating this way.
You should be able to make it work this way, but you'll just have to be aware of which fixture the individual units are coupled with. Otherwise, it's a crafty way to add more fixtures into the mix when the controller is initially limited to 16.
Hope that helps some.
Best,
-Tech