Not sure that is possible. You might be able to make a custom script that does this particular job. Keycloak has a REST API for admin functions, and I suppose OpenLDAP will have something similar. Biggest problem will be the passwords, because they are stored in a hashed format. Copying those may be figure-outable if OpenLDAP supports the same hash algorithm you are currently using in Keycloak…
Otherwise all of your users need to reset their password.