I tried this a few times but I cant do it :/

How can I replace boones quest reward red barret modell with a simple other hat ( the .nif files that must be manipulated? )

With GECK I only can choose from custom files that I dont have, do I have to extract the .bsa first ?

when I need to tell a world model to look to a different .nif, I open up the thing in FO3edit and put the path to the .nif to be exactly what I need. I don't know if it can be done in GECK, I have not seen a way.

I would. It's enabling. Personally, I don't use the archives so it's easier to tinker with stuff. You can always extract the archives and keep them handy just in case. I keep mine on an external and have all the .ini (GECK and game) bUseArchives lines set to 0 and have commented out the sArchiveList lines to keep my primary drive out of the red.

I tried Tarrant's method, worked just fine! Boon's baret is now a pair of cool looking sunglasses