As previous answers said, this is not possible (that I'm aware of!). You can either put the button in the tree lines, but this will create a different button for every line of the tree or you can create a server action that can be triggered from the 'More' menu on the top using as argument the selected rows.

For doing the later, you do it like:

<record id="id_for_the_action" model="ir.actions.server">
    <field name="name">The Name of the Action</field>
    <field name="type">ir.actions.server</field>
    <field name="model_id" ref="model_your_model"/>
    <field name="state">code</field>
    <field name="code>self.function_to_run(cr, uid, context.get('active_ids'), context=context)</field>
</record>
<record id="id_of_the_action_value" model="ir.values">
    <field name="name">Name for action value</field>
    <field name="action_id" ref="id_for_the_action"/>
    <field name="value" eval="'ir.actions.server,' + str(ref('id_for_the_action'))"/>
    <field name="key">action</field>
    <field name="model_id" ref="model_your_model"/>
    <field name="model">your_model</field>
    <field name="key2">client_action_multi</field>
</record>

Be aware that when putting model_id you need to put model_your_model with the model_ part included and _ instead of .

Hope this suits you!