Do you mean extensions? They're like just collectibles but they can also affect your account. Depending on the one you get, you'll most likely be able to set up it in your inventory.
Yeah if you buy an add-on and make it active in your inventory then it'll work (for instance, if you buy the avatar width extension but it's not active then it won't work, it needs to be marked active to work). This is why they made it so you can hide collectibles. I have my avatar width extension add-on marked active and hidden so that I can use it without it showing up in my lineup.