Is there a way to export list of users from Splunk and import them (along w/passwords)?
Once a user has been manually added to Splunk, can they be converted to a user using LDAP instead of "Splunk" authentication system?
Check out this previously answered question:
http://splunk-base.splunk.com/answers/11457/import-export-user-data
On the LDAP question , you would have to manually setup that user in LDAP and setup your LDAP Group to Splunk role mappings.
Also bear this in mind (copy/paste from Splunk docs):
Usernames in Splunk's built-in authentication system always take precedence over the same usernames in LDAP. So, if you have converted from Splunk's built-in authentication system to LDAP, you might need to delete users from Splunk's built-in system to ensure that you're using LDAP credentials. This is only necessary if usernames are the same in both systems.