It's pretty simple to whip up a custom door way mod to go from the Manor in Solstheim to your other house. Just make sure you use an empty spot in a cell that 'exists' in the house you've built. Doesn't even have to be a 'door' persay. You can set up any sort of activator as a door I believe. A magical portal thing isn't that hard to do. (I believe you can even use things that aren't normally available, but I haven't done that in a while.)