Do I need to recertify a product if I add a DC to DC converter?
First of all you have to re-verify your product to check if it has higher EMI emissions or not. If not, no re-certification is necessary provided you comply with the required documentation for such verification (Section 2.1043). Otherwise, if the DC/DC converter is incorporated in the circuit of the original product, you have to re-certify. 

If the DC/DC converter is a separated box and connects to the power source and your product through wires with connectors) either it is already certified by its manufacturer, or you have to certify it yourself, apart from the already certified product.

https://transition.fcc.gov/bureaus/oet/info/documents/bulletins/oet62/oet62rev.pdf