This article will guide you through migration from Joomla! to Drupal . Before you do migration you must understand some differences between both to make sure your migration to be successful:

Joomla! vs Drupal

1. Joomla only supports one Section and one Category for each content, while you can assign Drupal contents to several Sections/Categories.
2. Joomla does not support multi-site setups, so the migration must be put into a certain site if you already setup a multi-site with Drupal.
3. In this guide I assume you have a forum in your Joomla site. Drupal has built-in forum discussion, so you don’t need to install additional modules.
4. The term Blog in Joomla is not same as blog in Internet dictionary. ‘Blog’ term in Joomla is actually a teaser view of contents containing: Title, Introduction and a Read More link. So, in short, ‘Blog’ in Joomla terminology is not ‘Weblog’! If one is asking if Joomla supports a ‘Blog’ by default, then the answer is yes, but with a different meaning.
5. Comments on contents are no

Let’s start!

Components and plugins

Components and plugins cannot be moved. There is no such necessity. As a rule, you can always duplicate their work with Drupal modules. Thus you need carefully learn Drupal architecture to use  the engine’s abilities with a maximum performance.

Joomla Templates

Joomla-templates can be simply moved to Drupal.

Solutions

• Joomla module
• Joomla2drupal module

Joomla module

You can download this module here: http://drupal.org/project/joomla
It will help you to transfer the data to Drupal 5 only!!!

I couldn’t migrate my site using this module, but maybe you could. You can read the list of errors first: http://drupal.org/project/issues/joomla?states=all

Joomla2drupal module

It works perfectly!
The module imports data from Joomla-based site to Drupal automatically, but transfers the data to Drupal 6 only.
Here is the module description: http://drupal.org/node/249643

Using

The module will make everything by itself: create vocalubaries and terms, place your Joomla-articles to the respective categories etc.

The module is easy adjustable. It is enough to copy your configuration.php to the folder with module and it will read the necessary data to connect with database of Joomla-site.

Features

Keywords become taxonomy terms
This module will convert keywords from Joomla-site to terms (categories) and there could be a lot of terms! How to fix this: delete these terms directly from Drupal database, for example with the help of phpMyAdmin.

Articles Formatting
Imported articles appear in Drupal without any formatting. In this case, select the type of input in Drupal settings (I set php) and import (migration) articles again.

Attention

Create a backup copy of Drupal and Joomla sites befire importing!

Summary

Obviously that you should start with joomla2drupal module. if this alternative wouldn’t acceptable for you, use the first described module or find other solution.