A review of the latest report from BuiltWith®, the popular website profiler, reveals that the greatest migration of CMSs to the DNN platform is from Joomla to DNN, which may explain why you are reading this blog post. The answer to the question "How do I migrate from Joomla to DNN" -- as is the case with most good questions -- is "It depends."
DNN and Joomla are based on different software stacks, which may mean your hosting environment for the former may not be as strong for the latter. If you're using a 3rd party hosting service, be sure they understand DNN or choose a DNN-friendly host such as Appliedi or Managed.com (DNN Software also offers its own cloud hosting).
In all scenarios, you're going to need a DNN site to migrate to, and that includes needing a DNN theme. If you want the site's design to match your old Joomla aesthetic, you have two options: 1) Find a theme (formerly "dnn skin") in the DNN Software Store that closely matches your own existing one for a "close-enough- is-good-enough" result or 2) Hire a custom design firm to replicate your old look. Alternatively, you're always welcome to migrate your content into an all new look (which is not a bad idea if you're old site was not created with responsive design in mind).
Secondly, you'll need to transfer content (creating all new content means a new site altogether, so I'm not counting that approach). There are three ways to do this, manually ("cut and paste"), automated via a migration tool, and the more involved approach of doing a custom database migration. The manual approach is simple enough and reasonable to do if your page counts are in the low hundreds. For larger sites, Engage has developed a migration tool which can lift content from any CMS, including Joomla. While it makes short work of moving the content, particularly for large volume sites, it can't automate the correction of bad or incompatible HTML (bad data in, bad data out). That means another step of page clean up, visiting pages to make sure they're taking advantage of the new DNN theme's CSS and that the old content looks right in the new design. The last approach of doing a custom database migration can get fairly technical, but I'm including it here, since it is an option worth considering.
Finally, you'll need to account for functionality. Functionality, doesn't migrate. The extensions you have in Joomla won't replicate in DNN, so assuming you want to retain functionality you'll need to replace modules you had on the old site with DNN versions (3rd party or custom). The DNN modules will need to be configured to your site's needs, and concepts such as workflows will need to be configured.
That's it in a nutshell. No approach is rocket science, but none are entirely simple and quick either.
Are you moving from Joomla to DNN? Delighted to hear that. I'd enjoy hearing from you on what brought you to that decision and once you've accomplished the effort would love to hear how things went. Please share your comments below.