Im not to good with PHP and integration, or else i would do it myself. What would be a neat feature down the road, is if DirectAdmin could have a sign-up / billing program within it, like helm and hsphere.
This has been discussed several times before and you can certainly find other threads in the archives.
There are several third-party billing systems alaready available for DA; at least three that I know of.
I suppose you could argue that if DA added a billing system you wouldn't have to pay for a third-party system, but that's short-sighted thinking. The DA staff can't just keep adding complex subsystems to DA forever and maintain their current pricing.