You can convert a task to a project in Things, at least on the Mac. Move it from the Inbox to the Projects-Focus and it gets converted..
Projects with sub-projects can be handled via Areas of Responsibility. FYI I am using Things right now for two weeks and it works pretty well after a short adjustment phase. Some things do not work that well, but right now I am enjoying it.
Three things I really like about Things: the daily compilation of scheduled (which I use as start date) and due tasks. How it feels easy and fast to go through the next-list and add tasks to the today-list and the faster startup-time on the iPhone incl. the fast sync.