You can interact with SimpleAD in the same way you would with any ldapserver. You will need to be within the VPC but the following code works (assuming you already have the user Test Usercreated in the directory
Example
/tmp/username.ldif
dn:
CN=Test User,
CN=Users,DC=ad,
DC=example,
DC=org
changeType:
modify add:
objectClass objectClass:
ldapPublicKey dn:
CN=Test User,
CN=Users,
DC=ad,
DC=example,
DC=org
changeType: modify
add: sshPublicKey
sshPublicKey: ssh-rsa ....
ldbmodify --verbose --url
"ldap://ad.example.org" --user
"SIMPLEAD_USERNAME" --password
'SIMPLE_AD_PASSWORD' /tmp/username.ldif
This post details joining the realm and using it to auth the servers using sssd with AuthorizedKeysCommand.
https://github.com/localytics/chef-sssd/blob/master/GETTING_STARTED.md