I want to show user groups in selectbox rather than check box. below given is my code

<record model="ir.module.category" id="module_mat_mgmt_category">
            <field name="name">Materials Consumption</field>
            <field name="description">manage materials</field>
            <field name="sequence">46</field>
        </record>

  <record id="group_mat_mgmt_manager" model="res.groups">
        <field name="name">Site Supervisor</field>
        <field name="category_id" ref="module_mat_mgmt_category"/>
    </record>
  <record id="group_mat_mgmt_user" model="res.groups">
        <field name="name">Site Incharge</field>
        <field name="category_id" ref="module_mat_mgmt_category"/>
    </record>

    <record id="group_mat_mgmt_officer" model="res.groups">
        <field name="name">GM Projects</field>
        <field name="category_id" ref="module_mat_mgmt_category"/>
    </record>

This showing in check box, I want to show this on select box