Umashankar,

Modifying workflow is simply like you modify any of your xml records if we talk about your transitions and activities.

You should refer to the xml entities by sale.XMLID.

An example:

<record id="act_double_wait" model="workflow.activity">
            <field name="wkf_id" ref="purchase.purchase_order"/>
            <field name="name">WaitForApproval</field>
            <field name="kind">dummy</field>
        </record>

 <record id="trans_double_app_conf" model="workflow.transition">
            <field name="act_from" ref="act_double_wait"/>
            <field name="act_to" ref="purchase.act_router"/>
        </record>

You may even follow purchase_double_validation mdulel.

Thanks.