I've found a workaround for this topic, but I don't think it's the best solution.
As I said before, my first try was to create a Global Variable and associate the property that was linked with the Bill of Materials to it. It didn't work. For some reason, Solidworks don't evaluate the global variables contained in the parts by the time I open the drawing. However, if I create a Sketch with a single construction line and set the dimension of this construction line to be equals to the global variable, then it evaluates the global variable and my BOM shows the value I want to see.
The Global Variable I've created was declared like this:
"GROSS_WEIGHT" = ( 0.00000785 * "THICKNESS@SHEET-METAL" * "WIDTH@SKETCH_CROSSBAR" * "LENGTH@SKETCH_CROSSBAR" )
The problem with this solution is that I have always to create this equation and also a helper sketch to get the gross weight, but, at least, the gross weight is automaticaly updated.
Any other suggestions or upgrading for this?