Hi
There is nowhere presently where you can add an LTA-Airship rating. To obtain such a rating, you would have to be employed by an operator. To my knowledge, only John and Martha King of King Schools 'fame' have done such rating without being employed.
To be honest, there are not very many airships flying, the cost of operating such a large aircraft with its 17 man ground crew would probably be prohibitive to most. The company I flew them for had a 'company pilots only' restriction, i.e. we couldn't give instruction to just anybody.
May I ask why you want to add the rating?