What is the best way to add user points to existing users
One method here is to use the Userpoints API and execute a code snippet that will add the points for you. You can reference the userpoints API here. You can do something like this...

$initial_value = 10;
//@TODO: Build a list of all $uids in an array
foreach($uids as $uid) { =
  userpoints_userpointsapi(array(
    'uid' =&gt; $uid,
    'points' =&gt; $initial_value,
    'operation' =&gt; "Add Initial User Point Value",
  ));
}