Hello!
I had recently dived into the realm of 3-D modelling and animation, as that is what you will be doing. Specifically, I had tried to similarly edit a reload animation.
To my knowledge, through the G.E.C.K., one can unfortunately not add new animation choices for weapons, such as reloading. Basically, one can only change an existing one, that is, replace the animation file with a new one, which might not be a problem for you.
I myself was able to successfully modify an animation, such as the grip for a pistol in Fallout 3 (the 10mm non-scoped) and how the magazine escapes the pistol on reload.
EDIT: Haha, I forgot to mention I was not completely successful, however, the magazine escape did seem to work fine.
So, as an attempt by myself to explain the basics here would take a long time, I will provide you with all the links I used that should be of help to you (some I did not like, but maybe you will). As a note, you will be needing Blender for animation purposes (NifSkope you of course will also require), perquisite software described in a link, and, a lot of patience (this will not come quick). Furthermore, I would recommend simply learning how to use Blender first through the various wiki's and tutorials, as that would be best in the short and long run (in my opinion), rather than looking for your-need-specific guides, even though that is possible -- that's what I did, but, even though I don't regret it, I am stopping and going into the manuals when I have time in the distant future.
http://www.tesnexus.com/downloads/file.php?id=12248 -- I would recommend uninstalling all software stated here and reinstalling it as guided
http://www.thenexusforums.com/index.php?/topic/82738-fallout-modelling-mini-tutorial/
http://www.youtube.com/watch?v=hRcu7fbRBlA&fmt=22
http://www.fallout3nexus.com/downloads/file.php?id=4941
http://wiki.tesnexus.com/index.php/Avoiding_Blender_animation_pitfalls
http://www.gamesas.com/weapon-animations-t44320.html
http://cs.elderscrolls.com/constwiki/index.php/Exporting_Character_Animations_from_Blender
http://wiki.tesnexus.com/index.php/Creating_character_animations
http://www.fallout3nexus.com/downloads/file.php?id=6107