Correct me if I'm mistaken, but I think explorers guilds only give traits/ancillaries to generals/admirals to give them more movement points. As far as I know there is no way to increase the basic movement points, without adjusting them for all units. You can try to edit the unit type in EDU and make them infantry to see if they can keep up with the rest of the army. Movement points are fixed per unit type (infantry 100%; cavalry 150%; siege 75%).