If tapes are still active, you should be able to run bpimmedia command to list what's on particular tape.
If you have decomissioned media server only, but the images are still active in your catalog, you can find what media tapes were used and that's the only way to find out what's on the media you have from decomissioned server. It will not provide you the real data but can provide you some information.
If your existing server does not know anything about the media you are bringing from other/decomissioned environment, only way to read media is to import it and for import you do not need to do it twice like you have mentioned in your original post. You can import it directly to the NBU environemnt you want to unless you have different requirement or currect environment does not have drives for the same tape technology.