Electronic Alarms and Home Security Devices - Multiple switches or sensors to ONE zone

My installer put in my DSC2550 8 zone alarm - with one motion sensor, fire, plus doors/windows....my question is, one window was missed....can I add in a switch myself and run it to an EXISTING zone because all my zones are apparently used.
I am a complete newbie and will attempt this for the first time with caution....
Please respond in easy to understand language.
Thanks in advance for your help!!!
Pete

Simple. Assuming that all of your zone wiring is homerun to the main control location; take the lead that is landed on the zone input that _doesn't_ have the resistor off.

Pigtail one lead of your new zone wire onto this wire. Land the other of the new zone wires in place of the one you took loose.
(polarity does not matter).

That's it. Assuming that your contact is properly aligned and installed, you are done.

I've checked out the panel and it seems that all the incoming wires from the various zones are intertwined....and more complicated than I thought.
I don't see any resistors on the ends going to the panel (either the zone input OR the COM input for each zone)...
Each connector (triggers at the doors and windows) has a wire with 4 leads (only two are used per connector) - one should be for the zone and one for the COM. Looks like the installer, used the two remaining unused wires and connected them to another wire from another zone??

I guess my new question is: If I can run a fresh wire to the panel, can I simply insert one lead the zone and the other lead to the COM regardless of the existing wires coming into the same connectors? OR does my new wire need to be somehow connected to the existing mishmash of wires??

I hope I was clear enough to understand...

It's all good, that just means that the original installer turned off the resistor requirement in programming.

The same procedure applies, just ignore any references to resistors.

No. The way you are describing puts the new contact in _parallel_ with the existing zone. Both connections would have to be open at the same time for the zone to open.

All you do is take _either_ the zone _or_ the common off the terminal strip. Attach one of your pair from the new contact to that wire and put the remaining wire in its place on the terminal connection.

That's it, you're done.

Forgive my ignorance, but wouldn't that only connect my new contact to only one of the zone or common contacts? Doesn't the new connector have to have one wire in the Zone on the terminal and the other wire on the common on the terminal?
As I mentioned, I am a complete newbie at this?

(2) Also, does the power to the panel need to be OFF when connecting these zone wires?? I don't see an ON OFF switch, only the power wires going into the AC terminals - do I remove these first? OR is there no need ?

A zone with multiple openings is wired as a _series_ circuit of normally closed switches. Any switch opening activates the zone, that's why parallel wiring doesn't work (unless all switches are normally open, a completely different configuration).

As long as the system is not armed, you can monkey with the zone inputs to your heart's content. They are just switch inputs.

Perfect!! Worked like a charm. Thanks for all your help MrRonFL!!