I don't use any custom units since I am not doing projects that require it.
However, it is possible to create unit conversions using the Preferences>General>Unit Conversions dialog. These named units can then be used in Dimension Defaults, as well as in various dialogs. It's possible to set these custom units up so that they are the "Default Formatting" with specific 1000ths separators and decimal separators and unit names. When used in dialogs, the input will recognize the custom unit.
This is really a great way to customize Chief for other languages. Note that "Custom Units" will automatically display the units while the built-in units require the ".to_s" as I indicated previously.
For those who want to use these features you should search Help for "Unit Conversions" and "Number Formatting Class". It's a little complicated but once done it will be available for all Plans.