You will have to do this in online mode because of the nature of Weblogic's embedded LDAP. You can do this via the console if you go to:
Home >Summary of Security Realms >myrealm >Users and Groups>Migration tab
And do an export of the domain and import into the new domain.
This can be done with online wlst like:
connect('weblogic','weblogic', 't3://origDomain:7001')
domainRuntime()
cd('/DomainServices/DomainRuntimeService/DomainConfiguration/FirstDomain/SecurityConfiguration/FirstDomain/DefaultRealm/myrealm/AuthenticationProviders/DefaultAuthenticator')
cmo.exportData('DefaultAtn','/tmp/your.ldif', Properties())
connect('weblogic','weblogic', 't3://newDomain:8001')
domainRuntime()
cd('/DomainServices/DomainRuntimeService/DomainConfiguration/SecondDomain/SecurityConfiguration/SecondDomain/DefaultRealm/myrealm/AuthenticationProviders/DefaultAuthenticator')
cmo.importData('DefaultAtn','/app/userdata/abc.ldif', Properties())