You can import network device groups, network devices, user groups, users, etc. into ISE. The format is different depending on what you are importing. If you can get access to an install of ISE, just go to the section of the stuff you want to import and there is a button/link to generate a template to use for importing. For a users example, just go to Administration->Identity Management->Identities, click on "Import" and there will be an option to generate a CSV template. I imagine the hardest piece will be to export the users and their passwords from the old system. Usually you can get the users but not the passwords in an unencrypted form. So you may have to get the users to create new passwords in the new system. If you can get the passwords exported, then you can import them into ISE and that will save you a lot of headaches.