Is it possible to unlock a CDMA iPhone 4S (from Virgin Mobile) to use another SIM in it?
Of course, it's possible!  

Simply buy a Gevey Turbo SIM ultra S CDMA.  Note: type "geve" into google and watch it autofill with "gevey sim".  Good news- You're in luck because the price is not bad for the CDMA Gevey SIM turbo at this time. 

sometimes, they're close to $100.  only $35 right now.

also, i've included some other links to help explain

http://theiphonewiki.com/wiki/Gevey_SIM  (Personally been using iphonewiki for iOS firmware for years, totally legit.  NOTE: the reference to musclenerd possibly the most knowledgable mofo on baseband, unlock, JB etc.) 

http://www.applenberry.com/unlock-cdma-iphone-4s/