The Identity Manager AD driver matches against eDir users on the basis of the userprincipalname. However, this is an optional attribute that not all users (especially ones created before Windows 2003) have.
Here’s a quick one-liner I whipped up in a few minutes for a customer. This will find all users in an Active Directory tree that don’t have a UserPrincipalName, get their SamAccountName, build a UPN off of that, and inject the new UPN’s into AD.
ldapsearch -xWZLLL -D "cn=Administrator,cn=users,dc=example,dc=com" -b "dc=example,dc=com" -h myserver.example.com '(&(objectclass=user)(!(userprincipalname=*)))' samaccountname | sed '/sAMAccountName/s/$/\@example\.com/g' | sed s/sAMAccountName/userPrincipalName/g | ldapmodify -xWZ -D "cn=Administrator,cn=users,dc=example,dc=com" -h myserver.example.com