Since femanager is created with Extbase, it is easily extendable and I created a small TYPO3 extension named femanager_dmail_subscribe, that automatically adds "Newsletter subscription", "Newsletter category" and "HTML newsletter" direct mail fields to femanager, so editors can select the direct mail fields a frontend user should be able to edit.
|femanager plugin field settings|
In order to display direct mail categories in the frontend, it is required to add the sysfolder with the direct mail categories to the femanager record storage page.
|femanager plugin record storage page|
|Frontend user can edit direct mail fields|
I uploaded 2 versions of the extension to TER. Version 1.0.0 is compatible with TYPO3 6.2 and femanager 1.5.2 and version 2.0.0 is compatible with TYPO3 7.6 and femanager 2.x