When you move the mouse (x or y) you add a to a variable, say FVector2D FreeAim. (AddPitchInput perhaps)
Then in your viewmodel code, you can simply go.
float FreeaimMultiplier = 5.0f;
float Pitch = FreeAim.Y;
float Yaw = FreeAim.X;
Pitch *= FreeaimMultiplier;
Yaw *= FreeaimMultiplier;
SetRelativeRotation(Pitch,Yaw, 0);
This will make sure the weapon weapon model adds additional pitch/yaw based on the freeaim variable.