How do I use a DCMotor that isn't in WPILib?

Sometimes the motor you're using is so new that no one has PR'd it into WPILIb.

https://github.com/wpilibsuite/allwpilib/blob/main/wpimath/src/main/java/edu/wpi/first/math/system/plant/DCMotor.java#L166-L175

  /**
   * Return a gearbox of NEO motors.
   *
   * @param numMotors Number of motors in the gearbox.
   * @return A gearbox of NEO motors.
   */
  public static DCMotor getNEO(int numMotors) {
    return new DCMotor(
        12, 2.6, 105, 1.8, Units.rotationsPerMinuteToRadiansPerSecond(5676), numMotors);
  }

Creating your own DCMotor isn't too difficult and can be done easily if you use ReCa.lc to fill in the blanks

ReCalcwww.reca.lc

https://github.com/wpilibsuite/allwpilib/blob/main/wpimath/src/main/java/edu/wpi/first/math/system/plant/DCMotor.java#L45-L61

  /**
   * Constructs a DC motor.
   *
   * @param nominalVoltageVolts Voltage at which the motor constants were measured.
   * @param stallTorqueNewtonMeters Torque when stalled.
   * @param stallCurrentAmps Current draw when stalled.
   * @param freeCurrentAmps Current draw under no load.
   * @param freeSpeedRadPerSec Angular velocity under no load.
   * @param numMotors Number of motors in a gearbox.
   */
  public DCMotor(
      double nominalVoltageVolts,
      double stallTorqueNewtonMeters,
      double stallCurrentAmps,
      double freeCurrentAmps,
      double freeSpeedRadPerSec,
      int numMotors) {

For example a Minion would be

int numMotors = 1;
new DCMotor(12, 3.1, 200.46, 1.43, RPM.of(7200).in(RadiansPerSecond), numMotors);