I'm lost in the wording of your situation.
I have an iMic, and it has one output and input available. You can only have either the speakers or the input (i.e. a microphone or other audio recording device) active, depending on where the switch on it is positioned. If you have a computer lacking an audio output and relies on USB audio (examples: approx. 500MHz model PowerMac G4s, late graphite models) then you will need an iMic for speaker systems with only an audio plug. The iMic is also helpful for similar situations with audio in instead of audio out, as previously described, if you don't have an audio port.
Those are the only purposes of the iMic. So I'm lost as to what you mean by combining your speakers and another adapter with the iMic.