Measure up from the first hole (the one nearest the mouthpiece) 20mm and make a mark with a fine permanent marker.
Cover the mark with clear tape or tap very lightly with a centre punch to help the drill bit grip.
Drill a 5mm hole using very light pressure and a high drill speed as you don't want to buckle the tubing.
Measure down from the third hole (the third one down from the mouthpiece - the original third one, i.e. the fourth hole you now have in the tube) 14mm and mark again with a permanent marker and tape.
Drill a 5mm hole.
Now cover the original fourth hole with a bit of insulating tape (you may be able to get a bit the same colour as the whistle). This is a small hole anyway so its not much to cover.
Your whistle now has seven holes. Put your fingers on the top six holes and you should get a scale of E.
If you use your pinky to cover the seventh hole you will hear a concert D.
It may be safer to start with a 4mm drill bit and test the tuning as making the holes bigger sharpens the notes.
You could then file out to 5mm with a round needle file.
You can also give a few strokes across the new holes with a half round needle file to take off the sharp edges from the drilling.
The hole spacing is a bit irregular but it works.
Now t0 make a D whistle from a C whistle with a bottom hole for the concert C!