Since you also emailed this question to me separately, I'll post more of a response here:
You need to specify what exactly you mean. Are you talking about connecting a PIC to both an RS232 port, and the USB port of a computer? This would be no problem.
Or, are you talking about connecting a PIC to an RS232 port, and connecting some other USB device to the PIC, and having it act as a translator/adapter in between? This is NOT possible with a PIC, because a PIC CANNOT act as a USB host to another device, it can ONLY be a device itself. You'd have to be familiar with the details of USB to understand, but the job of a host is far, far more complicated than the job of a device. You would need to look to other, more advanced microcontrollers and probably some separate USB host controller IC's, and the whole thing would be beyond the realm of PICs.