How can I load a position without kings into an engine? I would like to be able to do load a position into a whatever possible (advanced) engine without kings. For example, the goal of the game would be to annihilate all the possible pieces instead of checkmating the king, and/or the king isn't even on the board. I sometimes test that. One example is when I want to see if a rook can stop two pawns at the 6th rank (it can't, I figured that out). Is there, maybe, an engine for that?
For most such questions perhaps a chess problem engine like Popeye can easily solve it.
However, the kind of goals your question is indicating like extinction of opponent pieces can also be set in configurable chess variant engines, e.g., Fairy-Stockfish or Sjaak II. I would assume that in most cases this will be the easier and faster solution, assuming that the problem is within the scope of the possible rule configurations.