We have a few linux users that prefer the power of writing their own procmail filters. Also we have some people with over 10 Outlook filters each.Is there such a thing out there that converts mail filters from XXX to sieve? What are the limitations of the web interface, versus what you can do with directly through via zmprov?
I could make something custom - maybe for Outlook do something in vbscript, then in Thunderbird and .procmailrc parse/convert it with whatever since those are just text files. Would probably be some quick-and-dirty solution, rather than a (probably more time consuming) generalized solution that I'd be proud to release.
Then again, I ought to take the easy way out and just tell users: "make your mail filters in the web interface (if possible). If not, let me know. It's not a big deal, you will just be doing this once."