lxml: add namespace to input file

I am parsing an xml file generated by an external program. I would then like to add custom annotations to this file, using my own namespace. My input looks as below:

<sbml xmlns="http://www.sbml.org/sbml/level2/version4" xmlns:celldesigner="http://www.sbml.org/2001/ns/celldesigner" level="2" version="4">
  <model metaid="untitled" id="untitled">
    <annotation>...</annotation>
    <listOfUnitDefinitions>...</listOfUnitDefinitions>
    <listOfCompartments>...</listOfCompartments>
    <listOfSpecies>
      <species metaid="s1" id="s1" name="GenA" compartment="default" initialAmount="0">
        <annotation>
          <celldesigner:extension>...</celldesigner:extension>
        </annotation>
      </species>
      <species metaid="s2" id="s2" name="s2" compartment="default" initialAmount="0">
        <annotation>
           <celldesigner:extension>...</celldesigner:extension>
        </annotation>
      </species>
    </listOfSpecies>
    <listOfReactions>...</listOfReactions>
  </model>
</sbml>

The issue being that lxml only declares namespaces when they are used, which means the declaration is repeated many times, like so (simplified):

<sbml xmlns="namespace" xmlns:celldesigner="morenamespace" level="2" version="4">
  <listOfSpecies>
    <species>
      <kjw:test xmlns:kjw="http://this.is.some/custom_namespace"/>
      <celldesigner:data>Some important data which must be kept</celldesigner:data>
    </species>
    <species>
      <kjw:test xmlns:kjw="http://this.is.some/custom_namespace"/>
    </species>
    ....
  </listOfSpecies>
</sbml>

Is it possible to force lxml to write this declaration only once in a parent el