I don't see why not, but don't ask me how as I'm not a builder. My guess would be either brick, poured concrete walls, or earth berm or dug in, maybe even a Monolithic Dome. Key area would be the roof and making sure it is attached to the walls more securely than in a conventional built home. As far as upgrading an existing structure, I doubt you could do it without a lot of expense even if it's possible. But you could always do like I did and build a shelter in your basement if you have one. I used solid concrete blocks for the walls as well as covering the 3/4 in plywood roof with them. I got the idea and plans for it here.

http://cache.viewimages.com/xc/3060812.j...55A1E4F32AD3138

Haven't had to use it yet, hope I never do but it's there if I need it.



Here is another idea.

http://www.rd.com/images/tfhimport/2000/May00_AskHandy/May00_AskHandy_TornDam_resize2.jpg


Plans for either a storm shelter or a fallout shelter would work.