Hi @dondavis, and welcome to the Forum!
You can normally edit the length of stems in Engrave Mode.
But: for your particular case you need to create “fake stems”, creating another 7:1q tuplet (7 quarter notes in the place of 1 quarter note) in a second voice, with the 2ns to 6th note (of this quarter tuplet) written on the lower G (then you can remove the first and last quarter rest), and with the Voice Column of the 2nd-6th 16th notes G of the original voice activated in the Properties, and set to 0 (so that the Gs align nicely). Then you can in Engrave mode, adjust the stems of the second voice. For very fine adjustments use the shortcut Shift+Option(Alt)+Command (Control)+ up and down arrow keys:
Result:
Dorico file example:
stems adjusted in a second voice.dorico (456.3 KB)
Here a video explanation :
(Add-on: you may also want to suppress the playback for the quarter notes. And of course you don’t need to input the rests 
)
[Sorry for undeleting my post @DanielMuzMurray , but I found a method to obtain what the OS requested, and I undeleted my previous post to expand the “stems prolongation in a second voice tuplet” concept]