For example is there any way to open a closed ticket?
No, and we probably won't add this. A ticket represents an incident/request, and should be closed when the incident is resolved or request completed. If the incident reoccurs, a new ticket should be raised.
The (kind of) workaround we added for this was the pending autoclose status.
Or maybe the correct way is to open new ticket and reference the old one?
Not currently, unfortunately. We do plan to add the ability to relate tickets to each other at some point. Additionally, I'd love to have the ability to track problems and changes in ITFlow - with tickets being attached to these. There's a Github issue open from a while back.
Also, is it possible to assign multiple technicians to one ticket?
No. I'm quite intrigued by this though, I've never come across a ticket system that allows this. Usually this would be achieved by "watching" or "following" a ticket so you get notifications about updates - again something else I'd like to add eventually.
is it possible to add more statuses to tickets?
We'll eventually have custom fields, including statuses, across the entire app. I know Johnny recently laid the ground work for this.