1) Ensure that the INPUT process does not specify a "Default Mode" 
   (which it always does)

 2) Ensure that the child spec does not specify an "Override Default Mode"
    (which it normally doesn't)

 3) Invoke the INPUT process with an invocation type of SUBPROCESS.  
    (--- DEFAULT MODE has a share class of SUBPROCESS.)

Think of INPUT Additional Attribute field "Default Mode" as "Initial Mode". Specifying "Default Mode" tells APPX to run the INPUT with that mode as the Initial Mode. APPX then ignores any value you might have set into the PDF.

If a parent INPUT process is running in Inquire or Change mode and you set the PDF to ADD mode before invoking a child INPUT, APPX runs the child INPUT process in Inquire or Change mode instead.

You can control the INPUT process mode by setting the desired mode into --- DEFAULT MODE in the child INPUT's Start of Process.