Well, you could read everything from the original InputStream. write it to a file, and then open a new FileInputStream to read the file you just wrote. In general that sounds rather silly and inefficient - why do you need a FileInputStream as opposed to an InputStream? But perhaps it might be necessary in some case.
It's also possible to create your own FileInputStream class which overrides all the regular FileInputStream methods and replaces them with methods that read from the other InputStream instead. This is probably a horrible, ugly idea. But it's possible.
I think that in all likelihood, you need to examine why you need a FileInputStream here. There's probably going to be a better way to accomplish what you're trying to do.
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