Unusable fuel is considered part of the empty weight for aircraft so when the aircraft is weighed the undrainable fuel is already included. Is there any time unusable fuel would be required to be listed either added or subtracted on a weight and balance form?
Since you've used the word "required", you may need a current specialist on the subject to get a truly valid answer.
That said, here's what I can tell you based on having written weight & balance software that was FAA approved for use at three different freight carriers in the period from 1988 through 2016 for B727, B747, DC-9, DC-8, and L-1011 aircraft.
That software made no provision for unusable fuel, as that was included in the basic operating weight. Also, the manual systems the software replaced had no provision for unusable fuel.
However, as I remember, the forms that came to me from the last weighing of the aircraft when adding an airplane to the program sometimes listed the unusable fuel, and sometimes the undrainable fuel as well, depending.
In other words, forms or software involved in the everyday calculation of weight & balance typically do not show unusable fuel. Forms involved in aircraft reweighing, whether due to maintenance or regulation, oft times do.
Edit - Seeing @Anilv's answer, I realized I may have misunderstood the question. I took "unusable fuel" to mean fuel that can not be physically delivered to the engines as opposed to fuel that can be delivered to the engines but should not be. Ballast fuel fits into that second definition. It physically can be delivered to the engines, but should not be. Using that second definition the answer to the question:
Is there any time unusable fuel would be required to be listed either added or subtracted on a weight and balance form?
is: Yes, there was an entry field for use for ballast fuel if such might be required to meet weight & balance requirements. On 747 freighters, for example, ballast fuel was on occasion used to move the zero fuel weight c.g. forward to be within limits. This was especially true for 747s that had been pax aircraft and had been converted to freighters. The addition of a cargo door well aft of the wing root produced a configuration susceptible to a c.g. that was aft of the aftmost zero fuel weight c.g. limit. The solution, if you couldn't arrange the cargo to take care of the problem, was to to put ballast fuel in the fuselage tank, which was the most forward tank.
While you were not supposed to burn that fuel, it was comforting to know that it could be used if you really needed it.
On the A310 freighter there is a 'Minimum Flight Weight' ie the take-off weight of the aircraft must be at or above this weight to depart. If the load is very light you need to add fuel (ballast) to bring the weight up to this minimum. On the loadsheet it is not included in the fuel figures but as correction to the Basic operating weight with the remarks 'Ballast fuel for MFW' or similar. This fuel is not usable as it would reduce the aircraft weight and bring it below the minimum required and therefore illegal.
Don't have the actual figures as its been 20 years since I worked with that particular type (and it was a b***h to get in trim too!).