I really do not understand, why would you again convert fiat to btc, because if they like to pay by fiat, that means the money is already in some form of other processor.

So converting to btc, it is manual task to you.

Or if you are saying that, you wish to convert usd to bitcoin, it is easy, you can have the option to pay in usd, but that will convert automatic to bitcoin, as soon as they donate to you.