This page includes a tutorial on how to add custom units.
Remember, that you can only create custom units in the activated version of the app!
Open Parsify Desktop and, using the top menu, go to View → Unit Manager.
Unit Manager will open. Type the name of your unit, then the definition (you can reference other custom units!). You can also configure the aliases which will be used to alternatively reference the unit.
Finally, click the "Add" button. The unit will appear on the right side if everything goes correctly.
You can edit the existing unit's definition by clicking on it.