[Samba] How to convert stand-alone samba servers to join existing Windows Active Directory domain

[tbuchanan at vinu.edu]

We have multiple stand-alone samba (4.2.10 and 4.10.4) file sharing 
servers with hundreds of local users on each server (not the same on all 
samba servers) in a CentOS/Oracle Linux (6 and 7) network.  We would like 
to convert these stand-alone servers to join an existing Windows based AD 
domain without losing data or ownership/permission metadata on these 
servers.  Is there a guide for doing so or can someone give the steps 
necessary to accomplish this task?  I see in the samba wiki how to set up 
samba as a domain controller or stand-alone server but nothing really on 
how to convert from stand-alone to domain member.  I am not real familiar 
with the Active Directory side of things so perhaps I'm not asking the 
right questions or looking in the right places.  Thank you in advance.