In Blender, no matter how I modify the unit model or hp_weapons, it is always in the same position in game. My problem is that in my custom unit model any weapon appears above the hand, not inside the hand but i cannot move the spawn point
You have to watch where the origin point of the main object is. Whatever the main object (that everything else is attached to) must be centered in the XYZ grid (basically should say 0.0 in all Location fields).
What you can do, and what I often do, is create a single "root" object with no vertices and have that grounded at the center, then attach the main weapon model to that, then you can move it around and make placement adjustments. Pull up any animated model (like a soldier) and it will have a "dummyroot" as the base "root" of all the attached objects.
The other option, is once you've moved your main weapon object, go to Object -> Apply -> Location and it should ground the origin point of your object to the 0 0 0 point on the grid.
hp_weapons does not do anything and was probably a leftover hardpoint from an earlier state of the game's development.