In OpenERP first, create your custom module and inherits the purchase module object like purchase.order object in your own module. Than add button in the purchase form.

Follow these step given below

Step1- First create module and than file like test.py file in your own module and pass this file in the __init__.py file.

Use this code given below

class set_product(osv.TransientModel):
    _name = "set.product"
    _description = "Set Selection"
    _columns = {
         'line_set': fields.one2many('prod.set', 'set_one', 'Sets Lines'),
         'sequences': fields.char('Sequence')
        }

    def add_items(self, cr, uid, ids, context=None):
        active_id = context.get('active_id', [])
        purchase_obj = self.pool.get('purchase.order')
        purchase_line = self.pool.get('purchase.order.line')
        set = self.browse(cr, uid,ids,context=context)
        order_id = purchase_obj.browse(cr, uid, active_id, context=context)
        set_line = set.line_set
        order_liness = []
        for line in set_line:
            line_vals = purchase_line.onchange_product_id(cr, uid, ids, order_id.pricelist_id.id, line.product_id.id,1,1, partner_id = order_id.partner_id.id)['value']
            line_vals.update({'product_id' : line.product_id.id,'name' : line.product_id.name, 'order_id':order_id.id})          
            ress = purchase_line.create(cr, uid, line_vals, context=context)
            order_liness.append(ress)
            line.write({'purchase_line': [(4, ress)]})
        return True      
class prod_set(osv.TransientModel):
    _name = "prod.set"
    _description = "Set"
    _columns = {
        'product_id' : fields.many2one('product.product', string='Product'),
        'set_one' : fields.many2one('set.product', string='Journal Entry')

        }

Step2- After this, create another file like test.xml file and pass this file in other file like __openerp__.py file. And create view using xml file.

Using this code given below

</openerp>
   </data>
      <record id="view_set_product" model="ir.ui.view">
            <field name="name">product_set</field>
            <field name="model">set.product</field>
            <field name="arch" type="xml">
                <form>
                    <group>
                        <field name="sequences"/>
                    </group>
                    <field name="line_set">
                        <tree string="Set lines" editable="bottom"> 
                            <field name="product_id"/> 
                        </tree>
                    </field>
                    <footer>
                        <button string="Add purchase Item" name="add_items" type="object" class="oe_highlight"/>
                        or
                        <button string="Cancel" class="oe_link" special="cancel"/>
                    </footer>
                </form>
            </field>
        </record>
        <record id="action_set_product" model="ir.actions.act_window">
            <field name="name">Set</field>
            <field name="type">ir.actions.act_window</field>
            <field name="res_model">set.product</field>
            <field name="view_type">form</field>
            <field name="view_mode">form</field>
            <field name="target">new</field>
        </record>
    </data>
</openerp>