I know its late but it may help for someone else.
One way to do this is to override whitelisted method erpnext.stock.get_item_details.get_item_details in hook.py of your custom app.
override_whitelisted_methods = { "erpnext.stock.get_item_details.get_item_details" : "link.to.your.custom_get_item_details" }
In your .py custom script file add this code
@frappe.whitelist()
def custom_get_item_details(args):
'that run standard event.
from erpnext.stock.get_item_details import get_item_details
out = get_item_details(args)
'Allow you to get data from args.
args = process_args(args)
'Your change event. You can append data to out.
return out
def process_args(args):
if isinstance(args, basestring):
args = json.loads(args)
args = frappe._dict(args)
return args