Hi Martin,
there aren't tools available for merge trace .db files similar to smartstor tools.
In APM project in which I'm involved, we store only the traces related to last 3 days (we consider TT as live or a"almost" live feature), I'm not sure that is so important have available a deep historical transaction trace collection to justify a merge operation if you are deleting a collector, for example.
Anyway you could open a new idea.