Yes, you would need to start again with Ripper, but not necessarily with the disc, i.e. if you made a VIDEO_TS folder or ISO from the disc, you can use that as a Source and select Direct Render for the sub stream that you want. This forum is to help you with DVDFab, no idea what other products may or may not be able to do. The page you linked was for adding external subs to an existing video file, not what you asked about. You can add external subs with Ripper for most profiles as shown in the capture below. The Add External also works in the Converter module, so if you have the subs in a file you could add them to an existing MP4, but I think you will get better results using Ripper with the Direct Render to Video option on the Advanced Settings page.
signals
Well, I'm asking about DVDFab solutions for the reported subtitles problem with that specific BD player and I'm glad to know it can add them in the SRT format. This way I can avoid the solution by which "the subs become part of the video and will appear without fail". I just mentioned the other product because I searched about SRT subtitles and it was the best result I found.
To test the suggested procedure involving a VIDEO_TS folder, I started to try with one such folder I happen to have. But two issues made me stop and come here to report.
1 - If I choose Audio > English AC-3/5.1 and hit the Preview button, the audio played is Brazilian Portuguese. And if I choose PT AC-3/5.1, the audio played is English.

Note: Maybe this can be an error that came from the original DVD, but I'm not sure.
2 - The subtitles don't appear in the Preview, whichever I choose from the list.

Note: There are two Portuguese options and I guess they are for Brazil and Portugal. I'll try to find my original DVD to check on this later.
I can't proceed with the task until I hear from you about these new facts. TIA.