P40/357 conversion uses same slide but different barrels for each caliber. Entire top end exchanges with P11 top end on same frame/grip. Cannot use 40/357 barrels with 9mm slide and can't use 9mm barrel w/40 slide.
The short answer is -- yes, you can buy a 40 or 357 conversion unit (slide, barrel, recoil guiderod and springs) that fits right on the P11 frame/grip. P40 magazines are not needed (although many believe they feed better) the 9mm mags usually feed 40/357 just fine but you do give up some capacity.
BIGIRON