Hi,

To add a new field you can  inherit the form and add a new field using xpath.

In Python :

First of all you have to inherit the model and add the  field to corresponding  model

from odoo import models, fields

class NameForClass(models.Model): 
   _inherit = "model_name"   

   new_field_name = fields.Char(string="Cost")

In XML:

<record id="give_an_id_for_record" model="ir.ui.view">  
  <field name="name">a_name_here</field>   
  <field name="model">model_name</field>   
  <field name="inherit_id" ref="external_id_of_original_view"/> 
  <field name="arch" type="xml">  
      <xpath expr="//field[@name='name_of_existing_field']" position="after">   
             <field name="new_field_name"/>      
      </xpath>  
  </field>
</record>

Have a look at this video too : Additional field to existing view in odoo 10

Thanks