Actually it IS possible, there is no simple way to do it, and it will cost you some $$$ to have it done at a facility - They will RIP the original menus and reconstruct them.
You can do it yourself, but it's a lot of work - you could use tools like vobedit and RIP individual MPG files or use MPEG video wizard and extract individual JPG frames for still menus, and using an advanced authoring software, re-create the links and buttons, sub-Ps, etc. Production company I work for uses Scenarist, I have used it myself to pull this off - Depending on the complexity of the menus, it could be a very long job - Which is why facilities charge a fortune
You could also use a tool like VOBLANKER/IFOEDIT to FOOL your DVD player into thinking it is processing 720x480 NTSC by changing information about the MPEG stream - You might away with it on some DVD players as people reported it works - However, not all players will accept this. Using this method (that you can try for yourself) you will retain EVERYTHING - there are other threads on this forum about how to do this.