You still need to add the location of the module to sys.path and then import it using the “import” statement. And I think you need to register the importer even before you open the multifile.
Maybe something like this (untested):
from pandac.PandaModules import *
from direct.showbase import VFSImporter
VFSImporter.register()
vfs = VirtualFileSystem.getGlobalPtr()
vfs.mount(Filename('foobar.mf'), '/mf', VirtualFileSystem.MFReadOnly)
sys.path.append("/mf")
import yourmodule