Techtrend,
Having been on the tail-end of the technology of dial-up modems and using software such as Procomm+©, et.al., I know your computer's software (Apple, Windows, Linux) has to interact with the outside world via either your computer's serial cable, parallel cable, USB cable, FireWire©, Ethernet, or some other carrier wire to a specialized compatible and accommodating peripheral device with your PC, much like modems, printers, and network gear (switches, routers) does today.
Excel in any of its 14 or more versions/permutations, to my knowledge, (maybe someone else on this blog can weigh in) was never designed to communicate directly with any outside peripheral device through any of the above named standard cables/ports and their protocols.
To solve your dilemma, you will most likely have to pay some technology expert (or Ninja; they like to use ninja here, so I'll throw that in also for effect) for
custom and specialized software written from the ground up for your computer to interact with your peripheral "time and signal" generator.
Did I communicate that clearly?
By the way, you never did mention what your time and signal generator is and how it functions...?