Summary
I ran across a few suggestions for outlines on Facebook that looked useful:

So I added them first to my copy of digitalCAT's Dictionary Maintenance Program:

... where when I want the steno for J to appear, I actually hit the J on my computer's keyboard.
Unfortunately, when I was adding that particular outline to Plover, I forgot that I couldn't just hit the computer's J key, so Plover locked up for several minutes before shutting down completely.
That's why making that particular field "steno only" might be a good idea.
Reproducing
Open a Plover dictionary
Add the pictured outline, but type the letter J instead of SKWR
Wait for either an error message (didn't happen) or Plover to crash.
plover.log
Plover Version
4.0.0.dev8+66.g685bd33
System
Mac OS X 10.10.5