We haven't tested the webdav function under dm Server, but we are not aware of anything which would prevent it from working.
As for the advisability of doing this in your existing dm Server instance, my main concern would be any contention between serving webdav requests and serving web requests. It's hard to predict whether there might be any performance bottlenecks, so I guess the best thing is to try it and see.
Of course, another downside is that if you do hit problems in a combined server, you'll always wonder whether they are due to some unexpected contention or perhaps functional interaction. So it's a trade off between maintaining a separate instance and potentially having these uncertainties.
Another obvious factor is if you need to recycle the one server instance for any particular reason - e.g. to upgrade - then you'll be forced to cause an outage for both webdav and web requests which may impact your users more than you would prefer.
Please post back here to let us know how you get on.