I often see people who cracked a WEP key and who wanted to convert it to ASCII.
My question is why do you want to convert it?
Maybe because it is easier to remember? Mmmh ... not always, especially if it's 13 random characters like this: $5@r6m2be_rEX
Maybe for network managers (command line/graphical, Linux/Windows)? No, they don't care if it's ASCII or hex. And an incomplete conversion would be unusable.
And, btw, if it was convertible, aircrack-ng would have given you the ASCII version :)