Adding Factions Collectors to Factions

If I already have Factions on my account but add a Factions Collecor's key, will I get 1-2 more character slots, or just bonus weapons and the minipet?

You'll only get the bonus that comes with having a collector's edition.

From the wiki:
Guild Wars Factions Collector's Edition (box content)

The Guild Wars Factions Collector's Edition box includes the above content plus the following collector's edition exclusives:

* Special In-Game Dance Emote for the Assassin and Ritualist
* Double-sided poster: Map of Cantha and Factions Poster
* Guild Wars Factions Art Book
* Guild Wars Factions Soundtrack CD
* Desk Calendar
* Mouse Pad
* Sticker Set
* Bonus Music Key
* Miniature Kuunavang

So, yeah it will give you the kunnavang and special dances in game.That's it.

Mouse pad? Anyone know what it looks like?
I would soo like to have that

The mouse pad is glittery, with that random assassin gal on it, with a purple background and "Guild Wars" emblazoned on top in orange.