It is not necessary for the database you are trying to import to include emails. Each member will be assigned a generic email upon import, along with a URL, Username and Password.
If you set your settings to auto-deliver leads, you may want to consider adding your company email to each member that does not have an email in order to guarantee leads are handled properly. By doing this, you’ll know for certain when a user tries to contact a member that does not have an email in your system.
When this happens, you’ll want to locate the email, update the email on the account, and send the lead again using the lead system. By doing this, you’ll have a great pitch when you call the company to let them know you've sent some business their way.