[SOLVED] Pre-Post animation spline NOT constant?

This is kinda hard for me to explain, but I try...
Is there any way I can set a animation spline to interpolate before and after animation graph to maintain a slope/speed instead of a constant?
in the picture, I have camera motion between frame 127-151. when I render the camera with motion blur(MB), the MB will look wrong because there is no before(frame 126) and after(frame152) animation to compute the correct MB.
I know I can manually create key frame to work around this, but shouldn't it be an option to set to have animation curve interpolate(Green Dash line) or Constant(Blue line)

Is there any way I can set a animation spline to interpolate before and after animation graph to maintain a slope/speed instead of a constant?
in the picture, I have camera motion between frame 127-151. when I render the camera with motion blur(MB), the MB will look wrong because there is no before(frame 126) and after(frame152) animation to compute the correct MB.
I know I can manually create key frame to work around this, but shouldn't it be an option to set to have animation curve interpolate(Green Dash line) or Constant(Blue line)