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 

1. Exit DRAW, run notepad and save the empty file to
   "%appData%\Corel\Graphics13\User Draw\GMS\UnitSwitcher.gms"
   COPY THIS EXACT LINE WITH QUOTES INCLUDED
2. Run DRAW->menu Tools->VisualBasic->Editor (Alt-F11)
3. in Projects pane dblclick GlobalMacros (UnitSwitcher), click Properties pane and change the name from GlobalMacros to UnitSwitcher

Paste the code

1. dblclick ThisMacroStorage inside UnitSwitcher (Projects pane), paste the code from my previous message

Create your own macros

1. press F2 to see Object Class browser, scroll to cdrUnit, lookup the exact unit name, it'll be cdrFoot
2. copy & paste as new line any of my SUB Unit...END SUB - and rename SUB and the unit afterwards like this:
   Sub UnitFeet(): SetUnits cdrFoot: End Sub

Assign buttons/hotkeys

1. [ 80 seconds of online 0.5MB video tutorial ]
2. [ step-by-step button installation ]
3. [ Macro button installer ]