Jousting is treated as a bonus action which does not cost the player anything, and does not strictly require making a melee attack. In order to joust, you must be 1) riding while wielding a lance, and 2) move in the same direction twice in a row with an enemy lined up at the end of the 2nd move. When this happens, the enemy is attacked automatically.

If adding a new centaur designed to be a significant threat increase over the existing ones, they should generate with good (if non-magical) armor and a good weapon, which is frequently a lance. If they have a lance, they will use it to joust you.

Jousting happens automatically when moving directly at a monster while riding and wielding a jousting weapon. You don’t have to be next to it to initiate the attack (and indeed, jousting will not happen if you start your turn next to the monster).

If you are a centaur (via polymorph or some variants’ centaur race), you can joust things without needing to ride another monster.

Rename the lance skill to “jousting”, and make lances not count as polearms for the purpose of applying them at range.

Jousting should be affected by riding skill, not just lance skill.