If you have been creating a number of installs on sub-domains within your hosting account and want to migrate them to a single instance of WordPress it is possible to reduce the load of supporting your sites by using a WordPress Network.
This process will also work for top level domains that are hosted separately.
There are some drawbacks before you get started and you should understand that combining websites into a network may require much more work then simply moving your sites between hosting companies or rearranging your domain structure.
Getting Started
1) The first thing that you need to do is make full file and database backups of the sites you want to combine.
Don’t just make a WordPress Export … you want to make cPanel downloads if you can and verify that the data has not been corrupted.
2) Setup your new install for your WordPress Network. If you already have an established site that you will use as the primary domain for the Network then you want to dump your cache, remove any unused themes or plugins and verify that all of the plugins that you are using are Updated and friendly with the current install of WordPress and also that they support Networks.
Unfortunately it is difficult to know if there will be any problems with a plugin
3) Use the internal WordPress Export tool to export all of your sites to xml files.
Keep your websites alive and do not delete the files because any linked media that you intend to import must be available.
See Final Note.
4) Use cPanel to create a wildcard sub domain for your primary domain. To do this simply add a new subdomain and name it *.yourdomain.com use an asterisk character for the sub-domain name then save.
5) Activate Multisite and select the subdomain not the subdirectory option.
6) install the WordPress MU Domain Mapper plugin if you are using more then one primary domain name on your install.
http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/
7) Create blog accounts for each site you want to install.
8) Verify your themes and plugins are compatible and copy any additional plugins as necessary.
9) Import all of your sub domain sites into their new blog locations.
10) Verify the setup of all plugins and themes. You may need to adjust and change a large number of settings to get your site looking as it did before.
YOUR SITES SHOULD NOW BE WORKING ON THEIR SUB-DOMAIN ADDRESSES…
11) If any of your sites need their own parent domain you can attach that domain now by using the SuperAdmin account.
Final Note
Expect to lose some settings along the way. The WordPress Export Tool is not always the best and you will need to verify all of your user accounts and setups.
It is important that you do not delete your installs as they will be needed during the import phase.
If you have problems with your user accounts you can try forcing a password update
http://wordpress.org/extend/plugins/reset-all-user-passwords/
Finally just expect problems .. you may not have any but if you approach this with the mindset that you need to be protective of your database, users and content then you will be prepared when things happen.
Visit every plugin page that you will be using and verify that people are not complaining about problems with MultiSite. Themes should work however remember to copy parent themes and children.
Become very familiar about user levels and what you can do with Multisite .. you do not need to make members of subdomains members of your parent site.
When you perform upgrades you will need to verify that each plugin has been updated on subdomain sites.
All in all this can be a difficult process and will require additional learning from a normal install.
What is the advantage? Yes that is the question. The primary advantage of a Network site is allowing memberships for bloggers however you can do this manually.
You should investigate how others use Multisite prior to going live with it.. as questions if you can and then set up a demo site for yourself.
