So the cassette adapter has a male 3.5mm plug on it?
You should still be able to use it with the device I pointed out by simply getting an adapter.
For instance something like this:
For only $0.38 each when QTY 50+ purchased - 3.5mm Stereo Jack to 3.5mm Stereo Jack Adaptor - Gold Plated | 3.5mm <--> 3.5mm Adapters
I'm sure you could also find the same thing at a Radio Shack.
Basically you want an adapter with 2 female 3.5mm ends. The cassette adapter plugs into one end and the blue tooth receiver into the other.