You already have an XRP address and wonder why you can't just import it into Payburner and instead have to create a new one. We know it isn't fun to have to spend an additional 21 XRP for a new account. But right now this feels like the right thing to do.
The short answer is that Payburner is a new service and we haven't earned the trust yet of the broader community. We feel that requesting you to enter an XRP address and secret might lead to suspicions that perhaps we are trying to steal funds. So we don't do that.