Actually, I think you can coerce extra CSS properties into a style class from the designer alone. Whether you’re supposed to be able to or not is another matter. 
A quick test in version 8.1.9 seems to confirm that a sort of “property injection” similar to the CSS injection hack @victordcq found will also work if you place your intended property within a property that takes a string value, you would just need to prefix it with a semi-colon to close the property you are entering it into:
; vector-effect: non-scaling-stroke
No need for a semi-colon at the end unless you wish to add multiple properties at once, since the originally edited style class property will end the line in a semi-colon automatically. Observe:
Inspecting an element that has been assigned this style class from Chrome’s web dev tools:
