odoo/addons/hr_expense/hr_expense_sequence.xml

<?xml version="1.0"?>
<terp>
    <data noupdate="1">
        <record model="ir.sequence.type" id="seq_type_hr_expense_invoice">
            <field name="name">Expense invoice</field>
            <field name="code">hr.expense.invoice</field>
        </record>
        <record model="ir.sequence" id="seq_hr_expense_invoice">
            <field name="name">Expense invoice</field>
            <field name="code">hr.expense.invoice</field>
            <field name="prefix">EXP/</field>
            <field name="padding">3</field>
        </record>
    </data>
</terp>