Not possible to perform dynamically, and relatively pointless as RP cannot be performed in-flight and the movement is completely outside of your control (and if two people talk at the same time, they will overlap unless some code forces a delay, etc etc).
Not very useful for roleplaying.
Your second line, however, seems to espouse something entirely different. What, an NPC that automatically mounts and set speeds a player with a specific mount? What if you're getting on horses, everyone's gonna get the same colour horse? It gets quite complicated fast to both set up and execute when you can just do .npc info on one of the mounts and then .mod mount into it. Or perhaps just allowing players to literally mount on top of NPCs by turning them into vehicles (which come with customizable speeds, too.)