Most steam units have a control wire that connects the control panel to the steam generator. You might find that modifying this would be harder and more expensive than just getting the proper speakers.
We just wrapped up a new Thermasol Steamer here in False Creek and I must say the unit is a sexy looking thing. Like a Red Ferrarri. One more coat of sealer and a little dry time and will put the new steamer through it's paces.
If you contacted your local rep you might be surprised to find that a simple AV cable or speaker wire connections are all you need. Remember that installing speakers insight your steamer will require special speakers and proper waterproofing / vapour proofing measures.