If your meaning a straight import, then ships are relatively easy. Altering .upc files and altering the .val file to be SH4 compatible via Silent 3ditor is fairly easy.
Importing the playable subs is much more involved, i.e. complicated.
"Operation Monsun" has all the U-Boats imported in, so thats already been done. IIRC, it's available in the downloads section. Also have a look in the SH4 ATO forum for the support thread.
Hope that helps,
Mike.