Yes, you can add the XP drive, as long as you don't expect to be able to boot in to Windows XP from it (because it won't).
If the drive has an IDE interface, and you are going to connect it to an IDE cable which already has the Windows 7 drive attached to it, you need to set the XP drive's jumper to "Slave".
But if the Windows 7 computer has both IDE & SATA ports, and the Windows 7 drive is connected to a SATA port, you don't need to change the jumper on the XP drive. Just buy an IDE ribbon cable and connect the drive to an IDE port.
If the Windows 7 computer doesn't have any IDE ports, your best option is to buy an external hard drive enclosure and put the XP drive in to that.
If the XP drive is a SATA drive, you don't set jumpers...