Within the Service Desk Manager's "Web Screen Painter" (WSP) there exists an option "Schema Designer". This feature allows you to create custom database tables and columns, which you can use within Service Desk Manager.
I've compiled a few useful documents for the Communities that provide some details on how the Schema Designer works, as well as some useful things to look into in case you run into an issue.
1. You can find the full documentation on the Schema Designer here:
There's a lot of information how to use the tool.
2. This document provides some additional information on some things to look into if the schema publish fails:
3. This document provides some specific details for using the schema designer in an environment with Advanced Availability enabled: