First of all, is your DTD standardized or proprietary?
If it is a standardized DTD, you are not allowed to change it. The LEP must be automatically generated by the stylesheet.
If you are using a proprietary DTD, change is possible, but it must be acceptable to others involved in the SGML/XML project. You want to avoid any DTD change that causes existing documents to be out of context and existing scripting and configuration files to be incorrect.
There are online tutorials on DTD development, including www.w3schools.com/dtd/. Also, books on SGML and XML cover the subject.