If the role needs to be added to every user, I believe you can use this script described in this article:

https://docs.servicenow.com/bundle/london-application-development/page/script/useful-scripts/reference/r_AddRoleToEveryUser.html

After writing this , you can then schedule this job to run every day after your LDAP integration import. 

Please mark this as solved if this solves your problem.

Thanks