How are you making the speed change? Are you using the time stretch percentage in the timeline or doing it by adding the Timewarp filter to the clip directly?
If you're time stretching the clip it should just automatically extend within your comp and you can enable optical flow by clicking twice under the frame blending icon.
If you're using the Timewarp filter you have 2 options. Option 1. Pre-comp your clip and leave room in your comp after your clip to account for the slow-motion and then apply the Timewarp filter to the precomp. Option 2. Apply the Timewarp filter to the effect directly and set your speed. Then in the Interpret Footage window change number of loops to 2 and you will be able to extend the outpoint of the clip so that you'll be able to see full clip length in slow motion.