We have a piece of data acquisition equipment (voltage data logger) that connects to a PC via a USB cable for power and communication. Because of some other equipment we already own, it would be convenient to have this connected to a serial port using RS232 protocol. Is there any easy way to do this (e.g. an off the shelf product)? I have found numerous USB to RS232 converters, but they seem to go in the opposite direction. Can I simply use one of these cables and splice in to provide external power to the unit?
USB to RS232 or TTL Serial are USB Slave/Peripheral Devices. They need a usb host to work. It does not work backwards by themselves.
Your three options are 1) if the existing device already uses a serial to usb device inside, you can modify it by removing that part and wiring to the existing serial connection. If the serial inside is TTL, you would need a TTL to RS232 level adaptor.
2) If you don't want to modify the device, and it is a simple CDC/virtual serial port device, a small linux computer with a usb and a serial port can be set up as a pass through device. Like 10~20 dollars and an hour to set it up if you have linux experience.
3) If the device is not a simple serial port device, requiring special drivers and software to work, then you are looking at a problem that will require a high number of man hours to reverse engineer and implement, if not money.
A fourth option is replacing the DAQ with one that suits your need.
No, you likely cannot marry the USB to RS232 converters. In the USB world, you need a device, and you need a host for the device (ignoring USB OTG for the moment). The converters are devices, and you'll have no host to control them with.
I suspect it will be easier to build your own voltage data logger w/ serial communication from scratch than it would be to buy or build a USB-to-serial converter. The biggest obstacle I see is that RS232 does not have power on the connector, so you'd have to power your device.
Alternatively, native serial voltage loggers are probably a dime a dozen! http://www.g4jnt.com/SerialLogger/HF-10.htm, for example.