You'd have to make the new field in the event details db and then you could modify the calendar_admin.pl file (or better yet, make a plug-in) to alter the add_event part of the code to populate that new event field with user data the same way the script adds the author name, author ID and time to the details when posting the event. The lines that do those can be seen below.
elsif ($in{'command'} eq "add_event") {
...
$properties->{'author_id'} = $User->{'username'};
$properties->{'author_name'} = $User->{'name'};
$properties->{'time_added'} = time;
...
JFYI
Dan O.