Yeah, they can be used for live. There are two ways you could go about doing that. Either with a digital console that allows for use of plug-ins (essentially, a live control surface. Those will cost you around $30,000).
Or the alternative is bringing a laptop to your live show, connect your vocal mic into your interface, run the interface into your computer, run your vocals through your reverb plug-in, then send the vocals AND reverb out of an ouput on your interface (or two outputs if you want a stereo reverb), then send that to the main mixer in the venue.
Yes, but why? To what purpose? There are easier ways to get reverb on a live vocal, I would think... but depends upon what you're doing, or trying to do.
The OP's question says for 'live singing', so I assume he doesn't mean monitoring. The only issue is going to be latency - as long as you can keep that to a minimum via system settings, you do it.