it's easy, here's the code, you can copy the example macros (SUB Unit.... END SUB) definitions using information from class browser in VBAEditor (F2 key) for "cdrUnit" class - it has all the definitions of units used in DRAW, just copy & paste.
Put the code either in new GMS (more reliable) or in GlobalMacros project's CorelMacros module
Sub UnitCM(): SetUnits cdrCentimeter: End Sub Sub UnitMM(): SetUnits cdrMillimeter: End Sub Sub UnitINCH(): SetUnits cdrInch: End Sub Sub SetUnits(ByVal U As cdrUnit) If ActiveDocument Is Nothing Then Exit Sub With ActiveDocument.Rulers .HUnits = U .VUnits = U End With End Sub
Create a new GMS macro container
Paste the code
Create your own macros
Assign buttons/hotkeys