I've been trying to give the tech lab addon to a building which wouldn't originally have it (not barracks, factory or starport). I haven't had a lot of success with it and was wondering if anyone else has done this successfully.

Interesting... I haven't looked at this in any detail and I am away for the weekend but I'll have a peek when I'm back. Should be a more or less simple 'come, see, conquer' case of copying how it works for the Barracks/Factory/Starport... I'd imagine it has something to do with placement, not only with the add-on but with the structure: example, lifting off and landing highlights the landing zones which already have add-ons available.