NTSC subtitles not synchron after reauthoring

Hallo, I would like to add an additional audio stream
to a commercical DVD preserving the original audio and
subtitles.
Seems to be easy:
1. Demux with PGCdemux
2. Author with Muxman
3. Replace movie with DVDremake

However the original is NTSC "drop frame".
If I try demuxing with PGCdemux and remuxing with
Muxman, even without the additional audio stream,
the result is not synchron any more.
Audio/Video is synchron, however Sutitles/Video
are not.

I guess that is due to the fact that MuxMan
recalculates the timecode into "non drop frame".
Is that correct? Or can I change the behaviour
of MuxMan? If not, I do have a problem.
Because the combination PGCdemux/MuxMan is the
only way I know of to use subtitles in the .sup
format directly.

What should I do in that situation?