A400 Xtreme Magazine Extension

I have just purchased a a400 Xtreme from Hunting & Fishing. It came with a magazine extension. The salesman was adamant that I didn't need a new magazine spring for the 2 shot extension. Is this correct?

I use the standard spring in my extension.

Yep, no need for spring extension

+1 It works fine

Great, thanks!

So to fit the extension do you just free up the spring by pushing in the holding pins and slide that into the extension or what do you do?