First, is this for XP or VX?
This would probably be possible via eventing. I would imagine you use terrain tags (XP) or check coordinates (VX). In a Conditional Branch Common Event, you would check if the player is pressing a directional key. If so, use another Conditional Branch to check the direction, and adjust speed values. If the player isn't pressing a key, set Direction Fix ON and move the player along the conveyor belt. It's a complicated process, but I'm sure it could be done.
If you'd like more details, just ask.