How to override table name andprefix in Magento model?

Question

I need to make Magento read/write data to an existing database table in order to integrate with another party system. The table cannot be renamed. If I specify it in the table tag in the model config as below magento adds the prefix. Is there a way to override the prefixing somehow?

   <models>
        <arithmetic>
            <class>Mcmr_Arithmetic_Model</class>
            <resourceModel>arithmetic_mysql4</resourceModel>
        </arithmetic>
        <arithmetic_mysql4>
            <class>Mcmr_Arithmetic_Model_Mysql4</class>
            <entities>
                <arithmetic>
                    <table>newslettersignups</table>
                </arithmetic>
            </entities>
        </arithmetic_mysql4>
    </models>