Element Domain Specialization Step 4: Integrate The Module Into a Document Type Shell
Step 4-3. Add .ent File to Shell
Step 4-5. Update domains attribute with new domain

Step 4-4. Update Type-Specific Parameter Entities in Shell DTD

Add the module type-specific parameter entities to the domain extension parameter entities. In concept.dtd, find the comment that says "DOMAIN EXTENSIONS". Update the %keyword; parameter entity declaration to include the %xml-d-keyword; parameter entity: 
<!ENTITY % keyword
  "keyword |
   %xml-d-keyword;
  "                   
>

This declaration has the effect of including all the XML module's keyword-type elements wherever keyword is allowed in the base DITA-defined content models.