it is possible but I do not know how. There is a Chinese SHIV fan who has added several SHIV warships to SH3; unfortunately I don't remember this guy's name. I do know that AG124 did some SH3 merchant conversions for SH4, so you might want to PM him and see if he knows how the opposite process works.
Semi - OT: You're on the MAW design team?
That's one great looking mod. I originally bought CFS3 just to play "Over Flanders Fields"; shortly afterwards I found out there was another mega-mod in the works.