I think in this game that pagan rulers should be made available for hire to act as mercenaries for rulers of a different religion. This could be done through the diplomacy menu of how the way you can ask allies to call to arms into your wars.

For them to join you in your wars will depend on far away your enemies are from them as well as how greedy they are for gold.

Depending on the size of the pagan kingdom will determine the hiring cost of them, as the gold will be paid to them immediately as soon as you call them into one of your wars. Indeed, hiring mercenaries from petty pagan kingdoms by Christian ones did happen historically throughout Europe and could make a nice addition to the game.