We don’t have a good way to do that, but that’s a good idea. You can import users like this: http://www.paidmembershipspro.com/importing-members-csv-simplified/
Here is a sample CSV file with a few random users:
http://www.paidmembershipspro.com/wp-content/uploads/misc/sample-data.csv
I believe we’ve used this site to generate that:
http://www.generatedata.com/
Our PMPro Toolkit addon also has some features related to this use case. For exmaple, it has scripts to scrub email address and other data of real users you import.
http://www.paidmembershipspro.com/pmpro-toolkit-add-on-for-sandbox-and-development-sites-running-paid-memberships-pro/