Is it possible (and if so how) to add data for a third party lens (Tamron 55-200mm) to a Canon 50D for peripheral illumination correction?
You can't load profiles of third party lenses directly into Canon EOS cameras. The only lens profiles that are allowed through Canon's EOS Utility used to load them are the official ones supplied by Canon and they only provide them for their own lenses.
The same goes for Canon's Digital Photo Professional editing application. In DPP, though, you can adjust the amount of peripheral illumination correction from between 0%-120% so you might be able to get fairly close with most lenses. Unfortunately, those options are all greyed out for images taken with third party lenses.
To do peripheral illumination or distortion correction with third party lenses you must turn to third party editors as well: Lightroom/Adobe Camera Raw/Photoshop, DxO, Capture One, etc. Those applications will allow using third party lens profiles if they are available from the lens manufacturer. Some will even allow you to create your own.