How to add a subtitle file while playing a video using Quick Time Player?
QT Player 7+ on Mac still supports .QT.TXT and .QT.SMIL caption files,
For more info, visit CaptionSync.
There is no straight forward way to add, but you can use third party apps like Subler which helps you to attach it directly to the file and it's very fast.
Just download the zip, extract it and launch it.
After launching the app, then follow these steps :

Open the app.
Go to File menu.
Select movie file of your choice.
Click on Open Button.
Drag your .srt file.
Click on Save.

Note: I did it for mp4 file.
I haven't seen any plugin so far for QT Player similar to VLSub.