Importing a single site Wordpress into a Multi-Site/Domain network

I have the Multi-Domain and Domain-Mapping plugins all set up good and we are looking to bring over a large single site into it.

I know that one way to import this might be to use the WP built-in Import/Export tools, but this is not working correctly and it is a very large file.

So, we need a way to import a single site into the Multi-site setup as a new 'site'/domain.

Surely there's a neat and easy way to do this? Is there a plug where I give it the admin user details of a remote Wordpress, so it can then import everything? (Including the users and their roles, etc)

Any assistance in this would be great. I'd rather not go about manually trying to shoehorn a single-site database into a Multisite db setup, manually altering tables and whatnot, as other (less MySql savvy) people need to be able to perform this task.