Answer
This was a known limitation that has since been corrected — the ability to modify what a built-in navigator entry points to (so it can be redirected to a custom lobby/page instead of the standard one) became available starting with IFS Cloud 21R1, and for the Apps10 release line, the same correction landed in UPD12. If you're still seeing the target page as non-editable in Navigator Designer, check whether your environment is actually on a release/update level at or after that correction — on an older release, this genuinely wasn't possible yet.