You can either add columns directly into the database, for example using mysqlman, or in admin->database->Category->properties. In the first case you can add the column wherever you like in the table but you will need to go to admin->database->Category->properties and resync the database to get the column to appear in admin. In the second case the column will automatically appear in admin but you will only be able to add the column at the end of the table.

As for the global something like this should work:

sub {
my $tags=shift;
if ($tags->{meta_name}){return;}
my $name=$tags->{Full_Name};
$name=~s,^([^/]+).*,$1,;
my $db = $DB->table('Category');
my $meta = $db->select('Meta_Description',{Full_Name=>$name})->fetchrow_array;
return $meta;
}