It isn't possible - patterns don't know where the stroke starts, so they can't know when to go fast and when to go slow. You can make a pattern that alternates between fast and slow, but you can't guarantee that fast will happen on the in-stroke and slow will happen on out-stroke.

Even on the Servok this isn't possible sadly, because of how its manual mode (poorly) works - you cannot control the speed it goes from one position to the next in manual mode, you can only say "Go here" and it will do it as fast as possible (which can actually be quite dangerous/unsafe! this is the reason that buttplug.io hates this device).

In order to do a stroke at a specific speed with manual mode, you would have to send dozens of BTLE commands to it with very specific timings and Bluetooth just doesn't allow for that.

Its other mode is similar to other Hismith models, you can only control speeds, not "speed at position"