You guys need to all be careful in whatever you are working w/ to import users now that dnn 3.3.x/4.3.x have released. Considering the API was rewritten I am not certain there is anything that exists currently that will work without flaws. Since the API is new as well, you really need to focus on the abstract provider to see how it adds users based on the currenly set providers in the web.config. Now, this affects Users, Roles, UserRoles, UserPortals, UserProfile, ProfilePropertyDefinitions, and some of the aspnet_ tables by default. See my blog post for more details, but I really think a new app that is a dnn module or a windows app which references the DotNetNuke.dll is probably the best way to make this happen. I mean, you still have to deal with encryption, and(because its a very new release) nobody has updated to the latest API yet in their modules.
I am just saying there is potential here for a disaster if one thing is not handled properly.