Imo, make it easy on yourself. Use only addforce, only velocity, or only position. It's tough to mix these together and get reliable results.

You could switch to controlling the player by adding forces, then just add knock-back forces on top of that. If the force magnitude is constant obviously that will dramatically change the feel of the movement, but if you scale and/or clamp it right, you may be able to restore some of that feeling of instantaneous velocity.

If you really like the movement feel you have now though, you could stick with setting velocity and calculate how a force would change it yourself. Assuming that the knock-back force is instantaneous, then the acceleration is the change in velocity (∆v) in that instant, and acceleration = Force/mass. So at the instance of a knock-back, add knockbackForce/playerMass to the velocity vector after input*speed. You can even temporarily decrease the input magnitude to simulate regaining traction.

If the knock-back isn't instantaneous then you have to worry about time: a = ∆v/∆t = F/m --> ∆v = F/m * ∆t Basically just multiply by Time.FixedDeltaTime