Can I add my own Javascript for Subtitles?

Hello,

I am trying to add my own Javascript to alter the subtitles in the video.
This is how I am thinking of doing it:

Change the subtitles to .JSON file.
Apply two types of filters.
Add Javascript to load subtitles according to how the filter is set.
Can I add my own Javascript to work on FV Player to achieve this goal?

I hope I made it clear for you to understand.
I look forward to your reply.
Thank you very much.