No tutorials that I'm aware of.
The batch import mode in the GUI is currently only available in Virtualmin Professional, so you'll need to use the command line option shown below (which is actually probably better for most cases where a lot of data is being dealt with).
The GUI batch import feature requires you to visit each virtual server and import each group of domains individually...so if you have hundreds of domains with one or two users each, you're in for a world of pain using the GUI. Anyway (for anyone following along that does have Professional), to get there, and to see the help, browse to "Edit Mail and FTP Users" for the virtual server you want to work on, and click the "Batch create users". Once on that form, you can click "Help.." to get complete information about the file format...it's basically a passwd-style file (i.e. colon-delimited list), though the fields are different to accommodate the various available features. You can leave fields blank, for options you don't need or care about.
The command line tool ''create-user.pl'' will allow you to write a simple shell script (or whatever language you like) to pull the data out of whatever your source is and act on it, based on whatever decisions need to be made to figure out which domain will contain which user. All of the command line tools now have good documentation thanks to a herculean effort by Jamie a few days ago:
http://www.virtualmin.com/documentation/id,virtualmin_api_create_user/I'm pretty sure all of the basic ''create-*'' commands are available in GPL. If they aren't, they should be...so let me know if you don't have it, and I'll make sure it gets into the next release (the rule we go by for the commands is: create, list, update, delete, actions go into GPL, while nothing more complex does...Professional has full command line coverage of every option in the UI in both command line and remote APIs).