Yes, there's a kernel interface (an IOCTL called RNDADDENTROPY) specifically for this purpose, it allows userspace (root user only as far as I know) to give the kernel random data samples along with an estimate of how much actual entropy the data contains.

Here's a bit of python code I wrote that could be adapted to do what you're asking for. It's designed to source random data from smartcards (via PKCS#11 libraries) at the moment but changing the data source to the output of another program would be trivial: https://github.com/infincia/TokenTools/blob/master/token-rng...

There are also a few mature projects designed to do this job (rngd and another one I cant remember the name of at the moment) that could also probably be made to work from arbitrary sources if they don't already, but if you wanted to do-it-yourself on the software side along with the hardware, that python up there would get you started.