iRacing-TV director has the ability to display subtitles. iRacing-TV will automatically record the start and stop frames of radio (voice chat) chatter. These will appear as items in the subtitles list. You can then edit these items by typing in what people are saying.

It's really a very simple process, but it can be a bit tedious. The most straightforward way to add subtitles would be to let iRacing playback your replay from beginning to end, so it can add all of the subtitle items to the list. Then you can rewind the playback to the beginning and start typing in what people are saying.

Here is a video demonstrating this process: