World Tools Pro enables most of the hidden Japanese typography functionality in InDesign, but as I discovered the moment I tried to really use it, they left out the ability to add custom ranges to composite fonts. The fix is to create the composite font as usual, then open the Adobe ExtendScript Toolkit, paste in the following snippet of code, edit as needed, and run it:

app.compositeFonts.item(1).compositeFontEntries.add({
    name:"Macron",
    customCharacters:"āĀēĒīĪōŌūŪ",
    appliedFont:"Minion Pro",
    fontStyle:"Regular",
    relativeSize:100,
    baselineShift:0,
    verticalScale:100,
    horizontalScale:100,
    scaleOption:false
  });

Set the value of item() based on font’s position in the pulldown list; the meaning of the rest should be obvious.