You can update a system variable. Here is a KB on how to do this.
https://www.qvera.com/kb/index.php/1188/can-i-programmatically-add-entries-to-a-table?show=1188#q1188
The system variable table lookup is designed for static lookups that have occasional updates. If you have a table that is constantly being updated you should use a database table instead. The database table allows you do run select and update statements for individual rows in the table. To update a system variable table, you have to retrieve the entire table, modify it and set it back.
Also, system variables are not thread safe. If you have "First in First Out" checked on the channel you can do this. If not then you can have 2 messages update the system variable table at the same time and lose one of the updates.