How can I introduce a unit in Scripter

How can I introduce a unit in Scripter?
In my special case I would like to add functionality which is contained in System.UITypes.
Is there a simple way to make all types which are defined in that Delphi unit known to the Scripter engine?

Anything from Delphi that you want to use from Scripter must be registered so scripter is aware of it, as described in this manual topic:

Constants, regular procedures, global variables must be registered manually as described above. Classes can have their methods and properties added automatically via RTTI:

You also have the option to use the import tool, which parses an existing Delphi unit and generates another Delphi unit which registers most of what it found in the parsed unit. But the import tool is a legacy tool and usually cannot parse some modern Delphi language constructs, so it's possible that UITypes might not be parsed. But you can always try to "fix" the unit source code by modifying things import tool cannot parse and make it pass.