there are other questions just off the back of that question, what level of integration are you looking for?
I'm guessing that you want them to look the same, at which point its just a matter of theming, WHMCS can be themed to what you need you just need someone to theme up WHMCS to match
if your not too heavily invested in WHMCS i'd suggest switching to HostBill its cheaper, slightly simpler and is much more secure!