Actually it you go to the black library forums they actively encourage people to write stuff like this. What's awesome is 3/10 times (a high number!) they are really good. I still remember an amazing short story about a psyker's experience as the black ship was about to deliver her to be eaten by the emperor.
As for fan fiction in general, I've written some terrible stuff before, but I've always acknowledged that fan fiction is the easy way out since A. You have a built in audience who you don't need to bother explaining things to, so half your work is done for you, and B. Its not canon so you can do whatever the **** you want, it's just more fun to make it appear as if it could be canon.
Yeeeeeeeep.