Unless it's already well in the works. Heck, even if Moffat is still working on the final script, he may have had a general idea in mind since the very beginning, and has shot additional material throughout his time as show-runner, for possible inclusion in this anniversary arc. That said, there may be scenes/sequences of the anniversary story that have been in the can for years.
Anything is theoreticaly possible but that doesn't make it probable.
The series has fallen further and further behind in production since Moffat took over to the extent that they'll only have shot one episode in eight months. The idea that they've somehow found the time and money to do all this secret shooting is wishful thinking.