/etc/security/user
default:
        admin = false
        login = true
        ...
        SYSTEM = "compat or LDAP" <-- this work
        registry = files or LDAP <-- Will not work of course, registry can only accept 1 parameter

if i loggin to test user, my AIX should search locally, if it doesn't find anything, it'll try on LDAP

# id
uid=0(root) gid=0(system) groups=2(bin),3(sys),7(security),8(cron),10(audit),11(lp),229(idsldap),15(dbsysadm)

# lsuser -a registry SYSTEM root
root registry=files SYSTEM=compat

# lssec -f /etc/security/user -s default -a registry
default registry=LDAP

# lssec -f /etc/security/user -s default -a SYSTEM
default SYSTEM="compat or LDAP"

mkuser test
passwd test
Changing password for "test"
User "test" does not exist.
Error changing password for "test".

grep test /etc/passwd 
test:*:210:1::/home/test:/usr/bin/ksh

# lsuser test
test id=210 pgrp=staff groups=staff home=/home/test shell=/usr/bin/ksh login=true su=true rlogin=true daemon=true admin=false sugroups=ALL 
admgroups= tpath=nosak ttys=ALL expires=0 auth1=SYSTEM auth2=NONE umask=22 registry=files SYSTEM=compat logintimes= loginretries=0 
pwdwarntime=0 account_locked=false minage=0 maxage=0 maxexpired=-1 minalpha=0 minloweralpha=0 minupperalpha=0 minother=0 mindigit=0 
minspecialchar=0 mindiff=0 maxrepeats=8 minlen=0 histexpire=0 histsize=0 pwdchecks= dictionlist= default_roles= fsize=-1 cpu=-1 data=-1 stack=-1 core=0 rss=-1 nofiles=-1 roles=

# passwd test
Changing password for "test"
test's New password:
Re-enter test's new password:

Connection      
su to local user         Works                    
su to LDAP user      Works 
ssh to local user       Works                    
ssh to LDAP user   Works 
mkuser                  Creates local user
rmuser                   Delete local user 
chuser                   Changes local user 
lsuser                    List local and LDAP user (or just the local users)
mkuser/rmuser/chuser -R LDAP works but my LDAP server will block this <-- this is the wanted behavior