The cache is stored under My documents\StreetDeck\Virtual Earth
There are 3 sub folders under that for Aerial, Hybrid, and Road.
Under each of those sub folders is a folder named after the numeric zoom level. i.e. (10, 14, etc.)
In the number zoom level folder should be the files named exactly as how they are downloaded from the virtual earth terraserver.
This post will show you how to download tiles from the terraserver based on lat / longitude.
StreetDeck will only download a file if it is not in the cache so if you put the appropriatly named files in to the proper directory, it will use that file. This also means that if you wanted to draw your own maps or use maps from someplace else, as long as you name them correctly, StreetDeck will overlay them in Navigation.