Unpacking/editing has mostly been covered in the Deluxe thread
, in short, you'd need either unlab and mklab from residualvm-tools, or bgbennyboy's
Resource File Creator and Dumper, he's even got a Grim Dialog Editor.
The datausr.lab is essentially a container which takes priority above the other *.lab files, so whatever you put in it will be used instead of the original content. I'm not sure how and where subtitles are stored though, so I don't know which files you'd need.
EDIT: Forgot to add, in order to actually use the datausr.lab you'd need to enable it from the GUI: Edit Game -> Engine -> Load user patch (unsupported).