You need to get OpenERP guide for Developers

this in you python code i think should work fine

 class product_product(osv.osv):
_name = 'product.product'
_inherit = 'product.product'
_columns = {
    'your_field': fields.char('Your Field Name',size=50,select=True),
    'your_field_2': fields.date('Your Field 2 Name', size=50, required=False),          
product_product()

this code in your xml should do it

<record model="ir.ui.view" id="view_product_product_form">
    <field name="name">product.product.form</field>
    <field name="model">product.product</field>
    <field name="inherit_id" ref="product.product_normal_form_view"/>
    <field name="type">form</field>
    <field name="arch" type="xml">
        <field name="name" position="after">
                   <field name="your_field" select="0"/>           
        </field>
        <field name="default_code" position="replace">
            <field name="your_field_2" />
        </field>        
    </field>
</record>