Yep, if you want to re-use your hard-wired sensors, it doesn’t make a lot of sense to hook them up to Z-Wave door/window sensors either. You might as well just replace them. Unfortunately, there’s no compatible multi-IO modules I know of. If you’re a DIY type, you could try using Smart Shield to hack something up. Another option would be to leave them connected to the alarm panel and integrate ST with the alarm panel. See this discussion, for example:
http://community.smartthings.com/t/dsc-alarm-integration-and-alarms-in-general/