In my document I have an appendix. This appendix has to be visible in my TOC and
in the Navigator.

Adding the appendix to the TOC is no problem:
For this I created an additional style based on Heading1 named "Appendix 1".
To add the "Appendix 1" style to the TOC I have added it to 
- Insert > Indexes& Tables > Indexes&Tables 
- check the box for Additional Styles > click on the Ellipsis button
- Add "Appendix 1" to level 1 > OK > OK

I am unable to display my Appendix in the Navigator inside the Headings-group.
Requesting an additional feature for this.