Templars will always avaliable for crusaders (unless you mod it in the cloumn 11), weather they're trainable or not.

Anyway, get Gnome, open up crusaders_unit_prod_11.txt, go to row #25 and column #50. Change ENGLISH to ALL_FACTIONS (though I prefer catholic only).
If you want them to be trainable to all catholic factions, just add CATHOLIC to the column 54