I would like to create a sul “D” etc string playing technique that has a dotted line and hook down, but I can’t find a way to add things like lines and hooks. Is there a way to adjust the sustain pedal playing technique symbol for this as can be done in the properties panel? (this would be done in Sibelius in the Line editing window).
I don’t think you can do this as a Playing Technique.
However to get this:
you just need to insert a pedal line and then set the following in properties (in Engrave mode)
Once you’ve created one of these, you can copy and paste or alt-click to anywhere else.
Yes, but I have to create it every time again in a new project. Would be easier if I could save it to the other defaults.
No you don’t - you can copy and paste it from one project to another!
Playing Techniques are unique to each project anyway, I believe, so it really wouldn’t make any difference to your problem if you could save it as Playing Technique.
That helps. But because it’s part of music engraving according to Behind Bars, maybe the team will add this to the arsenal 
Custom lines are on the roadmap, I believe, no need to keep hacking pedal lines.
Please refer to:
Making a new playing technique by using pedal marking has a side effect.