Solved using in my view the model product.supplier, inheriting the view product.product_supplierinfo_form_view, and making a new class on my addon based on pricelist.partnerinfo with a new field for the discount.

My class:

class pricelist_partnerinfo(osv.osv):
    _name = "pricelist.partnerinfo"
    _inherit = "pricelist.partnerinfo"

    _columns = {
        'discount': fields.float('Discount (%)', digits=(16,2)),
    }

pricelist_partnerinfo()

My view:

        <record model="ir.ui.view" id="purchase_discount_product_supplierinfo">
                <field name="name">purchase_discount.product.supplierinfo</field>
                <field name="model">product.supplierinfo</field>
                <field name="inherit_id" ref="product.product_supplierinfo_form_view"/>
                <field name="arch" type="xml">
                        <field name="price" position="after">
                                <field name="discount"/>
                        </field>
                </field>
        </record>