1a) Use SubRip to OCR the original subtitles into a text file
1b) Use Subtitle Workshop to convert them to .srt format
1c) Import the result into DVD-Lab Pro, and adjust as necessary.
2) Manually time them and type them in DVDLab Pro. This can be QUITE a pain, but it works.
In both cases, be prepared for SEVERAL muxes before it's lined up and ready. Subtitles are the biggest pain of all. Export your results into a text file and save it for future use if you reauthor again some day down the road.