"If the MS908S is able to program with a J2534 and an XP400, ??? .....  then what ?????      What was the rest of the question?

Well If you meant to start off your question with "Is" and not "If",then "if" you paired a new "Autel"J2534 Maxi Flash Pro or Maxi Flash Elite to a MS908S and the MS908S is within it's the free update period you might be able to program  BMW, Mercedes and possibly some VAG vehicles modules with updated calibrations from off the tablet for remainder of the update period, but once the Autel J2534 is paired to MS908S it permanently becomes a MS908SP and there's no going back , and updates for the SP are $350 -500 more pricey than the regular S updates.

  The XP400 is a key programmer or PIN extractor tool used in conjunction with the IM600. The IM600 will tell you when you need to use the XP400 and should give instructions steps for doing things. A MS908S software is different and lacks key programming and PIN pulling capability of IM608 and will lack instruction for XP400 , and unfortunately isn't going to become an IM608 simply  by plugging XP400 into your MS908S