Attach .idx/.sub files into an mp4 without OCR converting?

I have a bunch of .mkv files with external .idx/.sub files paired with them. I’m using subler to convert the .mkv to .mp4 for compatibility reasons but when I attach the .idx/.sub pair it wants to OCR them. I know that Subler does read and recognizes embedded VOBSUBs in mp4 files so is there a way to attach and embedded external VOBSUB/idxsub files into an mp4?