I have successfully imported my existing world, 'survival' and its nether, 'survival_nether'. There are already a bunch of Nether Portals in both worlds. I installed Multiverse-NetherPortals.

Worlds 'survival' and 'survival_nether' are successfully linked. When I go through a Nether Portal, it just takes me to a random spawn point (inside a wall, where I suffocate), not to the existing Nether Portals.