Adding a root note next to the key signature (like a single note Ambitus)

[Viktor Mastoridis]

Hello,

For educational purposes, I would like to see whether it's possible to add a 
small note head after the key signature?
Like a single Ambitus note, really.


Why would I do it?
For example, I would like to add a (small) D note to a G-Major key signature, 
suggesting that, despite the F# key indicating G-major or E-minor scale, this 
piece is in D-Myxolydian mode.
Viktor Mastoridis 📱