Well, the most efficent way would be to call out a new item, then assign properties to it.
weapon = RPG::Weapon.new
# Now, look in the help file under RGSS - Game Library, Data Structure, Weapon
# Simply use: weapon.attribute = whatever for all stats
Get my virtual database script.
Then, add your newly created weapon, and add it to data weapons.
Then, with the virtual database, use (I don't remember the exact syntax), something like:
$game_virtualdb.data_weapons = $data_weapons
It will save the weapon for just that save file into your database object for weapons.