You can add another spouse, but do not enter any marriage data, that will form a family that you can attach child to. If the biological father is not known, you can enter a special non-printing character for his name, (Alt - 0160) that is hold down the [ALT] key and, using the numeric keypad, type 0160, then release the alt key. it is a blank space, the program will object but accept it as a name. The printouts will still show him as a spouse, but no name and no marriage data.
But if you want to know when the program will say "Bastard child by ---" don't hold your breath.