Oh, it worked! So I can confirm that that process works for 0.92.
Since Inkscape won't even open for you, I would guess you somehow messed up something. What I did to edit, was just copy the last dash pattern (in text editor), then edit those dash code. I used the pattern that op was using (dash-dot-dot).
So I copied this, the last dash pattern:
<dash
id="dash-0110-0110"
style="stroke-dasharray:0.1,0.1" />
Pasted below it, and just above the group close tag (</group>)
Then for the id change 0110-0110 to 5-2-1-2-1-2, and for the style, change 0.1,0.1 to 5,2,1,2,1,2
Then Save the file and open Inkscape. The new dash pattern is right there at the bottom of the list (except for Custom, which I'm not sure what it is at all).