I would like to edit an item default blueprint without clicking 'edit copy' and without use hakpaks, is possible?
yes and I wrote the proccess somewhere already, think in PM
1) find the item resref (usually by placing the item on the ground and checking properties)
2) make a copy and change it the way you need, save and close item
3) locate the new item by its resref in the temporary toolset files (NWN/modules/temp0)
4) rename the file name to match the original item resref
5) open the file in GFF editor modified (seems this tool wasn't migrated to the new vault yet...) and change resref field to match original item resref, save, close
and thats the easier way as I already said '> there is an alternative way which is however unusable for anything that involves changing item properties (which is usually the reason why peoples want to do this in first place)