I am alittle confused by what you are saying. I am assuming you are talking about having it wired up so that the system has to be armed in order to purge. That would be the diagrams below. If you want to be able to purge with out activating the system just simply feed the purge button a keyed power with out going through the system toggle switch.
The purge should be wired completely seperate from everything else.
If it is the standard size purge no relay is needed. If it is our big show purge a relay is needed.
Below are two diagrams I happen to have on my laptop.
Standard purge.
Big Show purge
Let me know if you need any help.
Dave