You can really take two approaches IMO.
Either Buddhism is divine.
In this scenario the faith of the Buddhist person and their individual journey towards salvation (but with described using a Buddhist word) has a relationship with the divine that does not differ sufficiently, from that of any other divine religion to make it any different from the point of view of an outside observer. This has the big theological problem that the different divine faiths have mutually exclusive versions of what the divine actually is and what happens to the soul after death. But adding Buddhism to this theological pile of contradictions really changes little in the game.
Thus you can IMO add Buddhism to the divine with no real consequences for how the game works. (Note that this gets a lot harder if you try it on some of the Buddhist denominations, but the same problem, again applies to any of the other religions that are already included in the divine in canon.
This is the ideal solution if you run a game and for some reason you need Buddhism to exist in the world of your game but you dont want the game to be about what Buddhism is. The advantage here is that you dont have to bother with deciding what Buddism is on a metaphysical level or how to deal with the in- and out- of game consequences of it.
or: You can do something interesting!
This is what you do when you want to have a game where the exact metaphysical nature of buddhism is an important plot point in your game. E.g. perhaps Buddhist enlightenment is more akin to ascendancy into the hall of heroes and practicing buddhism allows a magus to become truly immortal by setting up shop in the magic realm. This approach is only suitable if you need buddhism to be something specific for your game to work and you know that your group will not be offended by it (preferably you would know so because you asked your group).