Hi,
The solution I would recommend and which doesn't require any code customization is similar to what you were describing on your first message in this thread: The MAD prices are displayed in the products listings and product pages, but then, during the checkout, it switches to the USD currency so that the PayPal payment method can be used with the USD currency.
If you want to keep your orders and notifications in MAD and only have the USD amount when the customer is redirected to PayPal, that would require modifying payment plugin so that it would convert the amount to pay back and forth between the currencies.
The modifications would be less significant doing it like that.
We don't allow for that as it is quite confusing to have one currency on the website and then another during the payment, and it would likely scare a lot of customers when being redirected to another website with a different amount to pay than what they were excepting at the end of the checkout, so I can't recommend that compared to what you're already doing. Of course, the best would be to use a payment gateway which supports the MAD currency. It would be even better for your customers.