SPCAF provides a Software Development Kit (SDK) to create custom rules.
Either you can contribute your rules in the SPCAF Community Forums or you can participate in one of the following open source projects:
SharePoint Code Analysis Frameworks Rules on OfficeDev GitHub
This open source project is a collaboration of the Office365 Patterns & Practices team at Microsoft and Rencore on the official Microsoft OfficeDev GitHub account.
The purpose is primarily, to create up-to-date migration assessment rules following the latest guidance of Microsoft, to transform farm solutions to the app model.
Contributions are welcome and will be awarded a free "time-limited" SPCAF Professional licenses. The rules are part of the official SPCAF releases.
To contribute you can use the free SPCAF Migration Assessment
SPCAF Contrib is an open source contribution project for SPCAF, lead by an enthusiastic crew of SharePoint developers from Australia and Russia.
The contribution project currently includes 100 rules, metrics, and inventory items for SPCAF.
To contribute please contact the project owners on CodePlex. Rencore has no direct affiliation with this project.