The way its done in the games core files is,
This line is commented out:
<IsAvailableForSovereignCustomization>false</IsAvailableForSovereignCustomization>
and this line is added:
<CustomizationPointCost>10</CustomizationPointCost>
IE:
<CustomizationPointCost>10</CustomizationPointCost>
<!--<IsAvailableForSovereignCustomization>false</IsAvailableForSovereignCustomization>-->
That is done for each item, you could copy/paste the core files into the items directory in your documents folder, and then modify it. safer then modding core files