User names are going to have to be populated from a third party source. For example with active directory there are a couple of ways to solve it. One is by linking syslog-ng to a lookup table and doing re-writes on the logs and append the user information there.
The other way is by doing the classic AD lookup table method.
Getting that information on all log types directly from Meraki is going to be very problematic without enhancing the logs along the way.