Purchase Order xml file override and add your domain and also python init file add depends purchase module.

Example,

<record model="ir.ui.view" id="purchase_order_domin_form">
    <field name="name">purchase.order.form.domain</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='pricelist_id']" position="replace">
        <field domain="['|',('type','=','purchase'),('type','=','general')]" name="pricelist_id" groups="product.group_purchase_pricelist" on_change="onchange_pricelist(pricelist_id, context)"/>
      </xpath>
     </field>
</record>

You can override search method of your target model. Then you can save existing target domain, remove from domain list. You need to delete because string '|' need to append in the begining of domain list. Append string '|' in domain list. After this you can append your desired domain tuple and the existing one sequentially.