Starting in Cortex-1.5.2-beta-b4666, you can now select one or more clips, Right-click and then choose Add Handles.
That will bring up this dialog, allowing you to set a number of handle frames to add at the in and out for each clip:
This is likely most useful for things like clip pulls from an EDL, but as designed it can be used regardless of whether the clips were created from and EDL import.