If you just have to decompress a single file, I don't see why this problem is impossible since you can tune your decompressor to that file.

For example, files containing the digits of pi or e will have a uniform distribution of bytes (and are therefore incompressible by exploiting redundancy) but it is very easy to write a "decompressor" for them as there is a simple algorithm to obtain these numbers.

Is there a simple algorithm for every possible number? Because then it just becomes a question of finding the algorithm that generates that number. My intuition tells me "yes" but that it may be very hard to find it. Plus, you don't need to find an algorithm for the whole file, just some subsection of it.