Re: convert 15 pin cable
No idea and I've never heard of a 15 Pin connector for scanners, most of them are 25 pin parallel, 9pin serial, USB, FireWire or SCSI. The only 15pin I know might just be the VGA port and that's usually used for video output.
*EDIT* LOL this brings back old memory, what you might have there is an AUI, it's a 15 Pin connector mainly used for the old 10Base5 network. So your scanner might actually be an old network scanner or if not it connects to an old sound card or with an AUI port. Some old sound card have an AUI port for joystick and gamepads, etc.
You might not be able to find an AUI to USB but there are AUI to RJ45 however I don't think it would work if you can't share the scanner through the network. There are old sound card that still have this port, I happen to have one lying around....but the AUI used for 10Base5 and the one for old joystick might be different. I don't know this.