Thank you all for your replies,
What I mean is exactly like what hunnaryb said.
In other words, the imagined scenario as follows:
Some websites offer instant Bitcoin purchase like Truecoin. If such website is embedded inside any merchant's website using HTML tags like <iframe>, the users will be able to get instantly their Bitcoins inside the merchant's website. Then the purchased bitcoins are forward automatically to the merchant's wallet to proceed the purchase. This process can make the users feel as if they used FIAT instantly to buy their goods.
This can be done with a developer. I'm asking if that can be done on the exchange level to look professional.
Thanks
First of all, I don't think there would be demand for such service. Most merchants (even if they are BTC enthusiasts) would prefer to keep most of their income in fiat, simply because they pay with fiat for vast majority of their operational expenses (stock, electricity, salaries etc). I can't imagine why any merchant would opt for instant conversion to BTC of all their sales income (well, maybe it would work if you operate in a country with hyperinflation).
The other thing is the risk of chargebacks. It's easy and safe to instantly convert fiat to bitcoins (i.e. as BitPay does it) as BTC payments are irreversible, but it would get more complicated if you try to do it the other way around.
But yes, technically speaking, it's possible.