iFlicks allows manually adding additional subtitles to videos using two different methods.
The simples option to add the subtitles is by dragging the subtitle files directly to the video in iFlicks.
It is also possible to add the subtitles by clicking on the + symbol in the lower left of tracks sidebar.
Once you have added the subtitles they will show up in the lower part of the tracks sidebar. Here you can set the language of the subtitles and the encoding if one or the other was not detected correctly.