Sorry, I must have misunderstood your requirements. I thought you were going to run tar on the PC and I was going to point you to the win32 version of tar. Instead, you want to run it on the MCU.
Surely, it's trivial to write a simple file concatenation program. You probably don't even have to do it. You could modify your file upload routine to upload a simple header (describing the files that follow) followed by the files one-by-one.