Below example will helps you to add the digital signature in OpenERP/Odoo, you just have to follow the steps mentioned below:

Step-1 Install the web_digital_sign module from Odoo apps to addons

Step-2 After installation you have to make user settings code like below:

from openerp import models, fields, api

class Users(models.Model):
    _name = 'res.users'
    _inherit = 'res.users'

    signature= fields.Binary(string=' Digital Signature')

after that we have to make .xml file for user interface..
<?xml version="1.0" ?>
<openerp>
    <data>
        <record id="inherited_res_users_form" model="ir.ui.view">
            <field name="name">inherited.res.users.form</field>
            <field name="model">res.users</field>
            <field name="inherit_id" ref="base.view_users_form"/>
            <field name="arch" type="xml">
                <xpath expr="//field[@name=' Digital signature']" position="after">
                <label for="signature_image" class="oe_edit_only"/>
                    <h2><field name=" Digital signature" widget="signature"/></h2>
                </xpath>
            </field>
        </record>
    </data>
</openerp>

Step-3 After that go to settings->Users->Preferences see below screen for reference: