Can "CustomCommand" ability be triggered by subsystem?

I’ve studied ship’s CustomCode for a month already, and recently begin to learn about CustomCommand. It’s quite interesting and I’m pertty sure that it would be useful on some of the FX:Galaxy Mod’s units, whose special abilities used to be triggered by defense field, kinda odd but that was the only way at that time. Now it’s different, with CustomCommand ability, I can do it a lot easier.
My question is, can this “CustomCommand” ability be triggered by subsystem, I mean, ship don’t have the ability until it build a specific subsystem, if so, how to do it? I’ve tried the old way, change DefenseField to CustomCommand, but it doesn’t work.