Yes, they have a converter but their API only exposes the ones they support trading for.
I tried to find out where their converter gets the data but it was too messy so I kept looking elsewhere.
Oh, turns out the Coinbase API does indeed support it:
So we can just use https://api.coinbase.com/v2/prices/BTC-ZAR/spot as for the other currencies!
So this is the place where one should look up such info: