Sigil is doing that correct. But there are a lot of converters or other programs doing it not correct.
Doitso: thx but this is only working for octet-stream, there are some other media-types out there.
@Diap: yes, thats what i'm looking for. But to achieve this i need to know how Sigil is doinf that to do it the same way. Now i need to have a look in the Sigil-Python-API to find out what is available to achieve this. Getting the fonts is easy, i've done that creating the Stylesheet. But now i need the way back
Edit: just read the API documentation and as far as i can see there is no method to write something to the manifest.