Geeezz, I guess I should have written the instructions out in full, I was just throwing a suggestion out there - if the OP'er had asked how to do it I would have explained in great detail how after I determined what their system was...
It was after my post that the OP'er verified their alarm system model, which is a different bird than most standard main stream systems (transformer or AC adapter is actually complete with wire and 2.5mm DC power plug or connector. You may know that most other systems have a set of screws that you connect your seperate burg cable to and extend as needed.
I used 18AWG as an example (thinking again it was a mainstream system), but now that we know which system we're talking about, and we see that it's 12VDC 100mA max, the OP'er could probably use standard 22 to 20 awg burg wire with a CMR/CL3R label which is available widely online and in most big box stores - and is rated for riser, which should be just fine as long as their not installing it in air handling areas like ductwork/return air paths or a known Plenum area.
The wording I used for my first reply which was:
You can add to the wire by splicing some 18 awg wire to it.
should have probably read:
You can add to the wire by making an extension cable with some 18 (or 20, 22) awg wire and a 2.5mm plug and jack.
Anyway, back to the OP, get yourself some 20 or 22 awg CMR/CL3R labeled 2 conductor wire online or from a local vendor/store, you'll also need an I believe - 2.5mm, right angle DC power plug (male end) and the mating female jack - both should be the "solder tab type".
Determine your run for the wire and pull your wire. At the control end install/solder the right angle end, and the jack end to the end by your transformer.
It would be recommended to make up the junction between the transformer wire and extension in a mounted low voltage box with a blank plate to protect the connection and any excess wire from damage or tampering.
That should get you code compliant wiring to the control, that will be safe, and durable.
Hope this helps clear it all up