In Odoo first, we create custom module and also create function and pass this function in button. And when we open the form we will select the product and this product will show in sale order line.

Follow these step given below

Step1- First we create test module and then create file like as test.py file and pass this file in your __init__.py file. Use this code given below

from openerp.osv import osv, fields
from openerp.tools.translate import _
from openerp import workflow
import openerp.addons.decimal_precision as dp

class prod_set(osv.TransientModel):
    _name = "prod.set"
    _description = "Set Selection"
    _columns = {
         'set_item': fields.one2many('set.prod', 'set_one', 'Set Item'),
         'sequence': fields.char('Sequence')
        }
    def add_item(self, cr, uid, ids, context=None):
        active_id = context.get('active_id', [])
        order_obj = self.pool.get('sale.order')
        order_line = self.pool.get('sale.order.line')
        set = self.browse(cr, uid, ids, context=context)
        order_id = order_obj.browse(cr, uid, active_id, context=context)
        set_line = set.set_line
        order_lines = []
        for line in set_line:
            line_vals = order_line.product_id_change(cr, uid, 0, order_id.pricelist_id.id, line.product_id.id, partner_id = order_id.partner_id.id)['value']
            line_vals.update({'product_id' : line.product_id.id})
            order_lines.append([0,False,line_vals])
            print line_vals , 'line_vlas'
        if order_lines:
            order_obj.write(cr,uid,active_id, {'order_line' : order_lines})
            
        return True

class set_prod(osv.TransientModel):
    _name = "set.prod"
    _description = "Set"
    _columns = {
        'product_id' : fields.many2one('product.product', string='Products'),
        }
set_prod()

Step2- Then we create test.xml file in your own module and pass this file in your __openerp__.py file. And use this code to open the form to click of button.

<openerp>
    <data>
        <record id="view_product_set" model="ir.ui.view">
            <field name="name">view_product_set</field>
            <field name="model">product.set</field>
            <field name="arch" type="xml">
                <form>
                    <group>
                        <field name="sequence"/>
                    </group>
                    <field name="set_item">
                        <tree string="Set Items" editable="bottom"> 
                            <field name="product_id"/> 
                        </tree>
                    </field>
                    <footer>
                        <button string="Add Product" name="add_item" type="object" class="oe_highlight"/>
                        or
                        <button string="Cancel" class="oe_link" special="cancel"/>
                        or 
                    </footer>
                </form>
            </field>
        </record>
 <record model="ir.ui.view" id="purchase_order_inherit">
            <field name="name">purchase.order.inherit</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
 <xpath expr="//field[@name='order_line']" position="before">
                        <newline/>
                        <group>
                            <button name="%(action_product_set)d" type='action' class="oe_highlight" string='Add Item Set'/>
                        </group>
                    </xpath>
             </field>
 </record>     
        <record id="action_product_set" model="ir.actions.act_window">
            <field name="name">Set</field>
            <field name="type">ir.actions.act_window</field>
            <field name="res_model">product.set</field>
            <field name="view_type">form</field>
            <field name="view_mode">form</field>
            <field name="target">new</field>
        </record>
    </data>
</openerp>