Hay,

I do object inheritance with _name != _inherit like in second part of doc

class custom_material(osv.osv):
    _name = 'network.material'
    _inherit = 'network.material'
    _columns = {
        'manuf_warranty': fields.boolean('Manufacturer warranty?'),
    }
    _defaults = {
        'manuf_warranty': lambda *a: False,
    }
custom_material()

Have you a tip to create a custom material from a network material (by copying it ?)