Yes, you can. From method of your button you can call your onchange method. Onchange will return dictionary, you have to fetch values from dictionary and pass that dictionary in write method.

Sample:

def your_btn_method(self, cr, uid, ids, context=None):
    for rec in self.browse(cr, uid, ids, context=context):
        #Pass required fields value as a parameter in onchange
        res = self.your_onchange(cr, uid, [rec.id], rec.field1, rec.field2)
        if res.has_key('values'):
            self.write(cr, uid, [rec.id], res.get('values'), context=context)
    return True

Hope this will help you.