See this: https://learnopenerp.blogspot.com/2018/10/how-to-add-custom-header-and-footer-in-qweb-reports-in-odoo.html

I did this exact same thing, if you still need to you can go to the drop down option -Change Footer and with Xpath replace the current footer and header with the values that you want, this way you can leave the original header and footer untouched.

Just paste this inside the <data> tag and add whatever you want

<xpath expr="//div[contains(@t-attf-class, 'header')]" position="replace">

        <div t-attf-class="header o_company_#{company.id}_layout">

            <!-- CUSTOM HEADER HERE -->    

        </div>

    </xpath>

    <xpath expr="//div[contains(@t-attf-class, 'footer')]" position="replace">

        <div  t-attf-class="footer o_standard_footer o_company_#{company.id}_layout" >

          <!-- CUSTOM FOOTER HERE -->

        </div>

    </xpath>