add more to starter content in unreal

can i add my own content and materials to the starter content, so every time i include the starter content i have my own assets made with materials

No, if you have assets you want to reuse you have to migrate the files over from a project that has them or paste the uasset files

Just as darthviper said, custom content/assets cannot be saved in the started content and cannot be included in the starter content folder.