You can import campaign map textures and things like tree models from FotS to base Shogun 2 (Sengoku) campaign, it nothing hard. Personally I suggest use Sengoku, because as I noticed FotS is a bit more problematic in modding (e.g. if you would use FotS, then would be best to move whole campaign to Sengoku campaign menu, because FotS has restriction here for playable factions).
About planning campaign. On "paper" I suggest prepare plan in the same proportion like available vanilla space (so 2x1 in base Shogun 2). This cause that will be not problem to fit all need parts of map to each other (so, map which you prepare in 3DS Max, and visual maps like heighmap, detail map (textures) and shroud (painted fog of war)). You can prepare plan in any resolution, just keep proportion. E.g. my plan has resolution 15024x7514, and base on this I'm doing everything. And in case of my mod this plan from start is prepared "artistically" because in last part I import this plan as shroud to game (shroud has resolution 32768x16384, but after upscaling my map still looks enough good)
Maybe I show here my work pipeline (even if I sometimes I do this thing in different order).
1. Plan of campaign map (is here every important thing, position of settlements, roads, rivers, region borders, forest, hills, mountains, etc. (you can also plan on start where will be mines, small towns, farms and ports) (I downscaled resolution here):
2. Heighmap (all space in heighmap file = all free space in 3DS Max file) - I use brushed from S2 and Attila TED to draw mountains, etc.
3. Detail map (here available space = playable area in DB files, personally I set playable area as x=-640x640, y=-320,320 even if I not use this whole space, just is easier to do detail map with this setting)
4. Map in 3DS Max (here I use all previous maps as references) - I show here only part of layers
5. Add plan of campaign map as shroud to game
6. Final effect (I missed here some steps like adding trees models, but it isn't tutorial)