Use LDAP and internal account system together

Hi,
I would like to use LDAP and internal account system together.
Now I have about 20 users in internet account system. I’d like to leave them there.

But for some new users I would like to use SSO (probably using LDAP).
Is there any way how to combine LDAP and and internal system?

For example: I have VPN server on MikroTik. Some users have local account. Other users are in Active Directory (using RADIUS). MikroTik first try to find user in local database. If user is not in local database, MikroTik will try to use RADIUS (Active Directory).
That is what I need.

Thanks.