i may be wrong here, but i think that if you run your 722 optically into your HD24, you will loose the ability to add more than the 2 channels from the 722 in the Alesis. In other words, if you have an optical signal going in and it only contains 2 channels, it will render your other 22 channels useless.
i'll give it a try next week and report back with my findings.
oh, and to answer your question, there is a $50 Hosa box that will do what you are looking to do.
722->coax output->coax/rca adapter->spdif cable->hosa box->toslink cable