Anyway to import User accounts?

Where are you importing it from?

And it’s easy enough via a custom PHP script.

Well all the employees are on a windows network, so ideally it would be Active Directory – or I could import it from a database or excel file

Exporting to an Excel file and then to a text file (tab seperated, space seperated, whatever) would be needed. Then just a script that parses the text file and inserts into the users table is all that would be needed.

Or perhaps just an LDAP plugin would do?

https://www.kaneit.net/ldap-plugin/

Oh, I had never heard of such a thing. Cool.