Hi,

create a new sequence :

Settings>>Technical>> Sequences & Identifiers>>Sequence Codes. then

Settings>>Technical>> Sequences & Identifiers>>Sequences.

then create a new module to add a Employee Code

class hr_employee(osv.osv):
    _inherit = "hr.employee"

    def _get_code(self, cr, uid,context, *args):
        obj_sequence = self.pool.get('ir.sequence')    
        return obj_sequence.next_by_code(cr, uid, 'hr.employee.sequence', context=context)

    _columns = { 'code' : fields.char('Code', size=64), }

    _defaults = { 'code': _get_code, }

 hr_employee()

you can define this sequence in your module :

my_module_data.xml :

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data noupdate="0">
        <record model="ir.sequence.type" id="hr_employee_code_sequence">
            <field name="name">Code sequence hr employee</field>
            <field name="code">hr.employee.sequence</field>
        </record>
        <record model="ir.sequence" id="hr_employee_sequence">
            <field name="name">Sequence For employee</field>
            <field name="code">hr.employee.sequence</field>
            <field name="active">TRUE</field>
            <field name="prefix">EMP</field>
            <field name="number_increment">1</field>
            <field name="number_next">1000</field>
            <field name="implementation">standard</field>
        </record>       
    </data>
</openerp>

and in __openerp__.py:

"init_xml": ["my_module_data.xml"],