I guess that it a matter of defining DOABLE.
It is doable to manually convert a script into subtitles for a video.
It is definitly a lot of work, because you need to make sure that the subtitle is:
- starting at the right moment
- ends at the right momnet
- Is not too long to read in the time give
- (and for PD) does not overlap within 2 secs for the adjacent
For a short video this is all doable, even if you manually type-in the text.
For a long video it is hardly doable, look at:
- convert the script into a flat text file
- prepare the lines such that they fit the screen
- and give this file the right "format" that can be understood by PD, like
1
00:00:00,0 --> 00:00:02,0
text a
2
00:00:16,200 --> 00:00:18,200
text b
You can also try using a subtitle editor. Like: https://www.nikse.dk/
And there are some video to subtitle services and software. Try to browse for these.

When done you may want to check your subtitles, whether they are in sync. Moving the subtitles around in PD is doable. But it will take time....
Maybe that will make the process a bit lighter. But for a long video manually, doable??