Ok, I figured it out. I decided to use the internal API to make the changes. Here is the code I'm using if it can help anyone else.
add_hook(
'ClientAdd',
1,
function($vars) {
$userid = $vars['userid'];
$command = "updateclient";
$adminuser = "adminuser";
$values["clientid"] = $userid;
$values["customfields"] = base64_encode(serialize(array("1"=>"CustomValueStringToSave")));
$results = localAPI($command,$values,$adminuser);
return array();
}
);