After a bit of research in the source, you can modify the field it reads inside of Hercules's /src/login/account_sql.c at line 525:
"SELECT `account_id`,`userid`,`user_pass`,`sex`,`email`,`group_id`,`state`,`unban_time`,`expiration_time`,`logincount`,`lastlogin`,`last_ip`,`birthdate` FROM `%s` WHERE `account_id` = %d"
just change `group_id` to `level`.
It should be around the same place in eAthena (and other) source if you want to change the other emulator query instead.
JoWei got a reaction from Mumbles in Merging with existing eAthena login
