From 2gang to 3gang light switch!!!

I am looking to install an outside wall light, however to do this I need to wire it up to a power supply in my hallway preferably. Ideally I would like to replace an existing 2gang light switch (which controls the landing and hallway individually) with a 3gang switch, and incorporate the out side light to be controlled from this point also. However I am a novice at this sort of thing and have no clue if this is possible or how to do this, please could someone/anyone give me some advise?

You'll find all the info you need in the For Reference section.

The key thing to realise is that there is no neutral at your switch, so you cannot make the outside light work by just running a cable from there - you have no alternative but to break into the downstairs lighting circuit. Hopefully from under the floorboards upstairs, as that's a lot easier than via the downstairs ceiling...