a julian date-format is currently unavailable as of Opus v12.3 build 8360
You can already use any date format you want, unless you're just asking for "jjj" as a shortcut for one particular format.
Edit: Actually, maybe you can't as there isn't a day-of-year code, only day-of-month, week-of-year, and similar.
But you could use scripting to do the same thing. I am not sure whether or Windows has built-in support for a day-of-year code in date formats, and no one has asked for it before (while people are already confused enough about things like ISO year vs normal year), so it's probably not something we'd add ourselves on top of the standard codes.
Fun fact, the JD changes at 12:00 pm.
There also seem to be many different "Julian date" formats, not just one.
Seems something best left to scripting, so people who need it can get exactly what they want.