In TYPO3 6.x BE, how to enable another doktype's fields in a sysfolder

In TYPO3 6.x BE, how to enable another doktype's fields in a sysfolder

In TYPO3 6.x, I have a sysfolder where I would like to enable the
"abstract" and "media" fields in the Backend. How can this be done?
PS The reason for is that I need their contents cascading to subpages. But
the sysfolder/page they're on can't be displayed in the FE, so I want to
prevent editors from adding content.