You don't want to do that by the way. You want to create a MOD and inject your changes into that.
Edit: Try this @Stormie97 and see if it works (you'll need to add your item's template name and compile it):
void main() {
object oStore = GetObjectByTag("m_202_001", 0);
// To make sure the item only spawns once, check Dendis for the presence of
// a custom local boolean we'll create. If it returns false, spawn the item.
// Since the DLG we are firing the script from is owned by Dendis, we can use
// OBJECT_SELF for any call that requires a target object, like the boolean set/get.
if (!GetLocalBoolean(OBJECT_SELF, 66))
{
// The check returned false, so set the boolean to true so that the
// item won't spawn a second time on future triggers of this script.
SetLocalBoolean(OBJECT_SELF, 66, TRUE);
// Create the item in the store's inventory. The values are of the form:
// Item template resref (string) / target (object) / item stack size (int)
CreateItemOnObject("item_template_here", oStore, 1);
}
}
After compiling it, you'll also need to edit Dendis's DLG and add the script name to one of the nodes. I would put it in the Script #2 slot on Reply 9 ("Just let me shop"). Just make sure not to replace any existing script.