I don't think you can do it in that way - you have to have the whole map in at the same time - to avoid scaling and allignment issues - and i dont think you can do that in Bryce - you need to use a 3D modelling program which can "cut" and "Loft" your map then export each "chop" as a seperate waterfront or 3DS object but in one file - both waterfront and 3ds files can contain more than one object/mesh  - i think C4D and 3DMAX is able to do these things - maybe a guru here at r'osity knows how to  do that in one of the forementioned proggy's

Regards
Arne