Try this:
In the player's Step Event, include this piece of code, or similar:
if collision_line(x, y, x, y + 32, objConveyorBlock, true, true) {
hspeed += 0.75;
}
You would have to distinguish between left and right conveyors eventually, but this should be a start. Also note that the player's hspeed gets reset at the beginning of each step, so you need not worry about the player accelerating to the right.