Mass Import of Users

Unfortunately, no; there is no bulk import.

Do the 300 users already exist in wordpress as local users? Or are these going to be 300 new directory-authenticated users? If it is the latter, then if you have these 300 users in an AD security group then you could limit authentication to the security group (Authentication Groups setting in the wpDirAuth settings) and then enable Automatically Register Authenticated Users? in wpDirAuth.