If I have an objects track and orientation in an external text file(CSV). How do I get it associated with a layer in Hitfilm?
HitFilm doesn't have support for importing such data, so not much luck here. You can however try to convert that csv data into a HitFilm composite shot, as those are saved in xml. That's the way mocha exports to HitFilm (if mocha can import csv data, then that might be a way to got), and also a plugin that someone wrote to get camera and null data from blender into HitFilm.
Hitfilm can import tracking data in .ma format as well. I think there are csv/ma converters.
Oh right, totally forgot about that. Ahem...
Well, that's a Hitfilm 1 feature, but with HF2 and 3 having mocha, who needs to import tracking data? (Other than users migrating between 3D software)