The Wizard tells me: "You are trying to add a virtual device of type 'stripe' in a pool that has a virtual device of type 'raidz' "
This may be a safety feature that was added to freenas. If you search through the forums, there are multiple instances of users accidentally, and irreversibly striping in a single disk vdev and destroying their pool's redundancy. So, maybe the developers have tried to save users from themselves.
I'm guessing you don't have to have a vdev of the exact size and type as already exists on the system in order to stripe it in... just something more than a single disk. If you really want to stripe in a single disk vdev and destroy your pool's redundancy, you can probably do it through CLI.
You may know it, and probably don't want to hear it, but it sounds like your setup has a lot of problems that may result in data loss. First, you have a single disk vdev in vol2 which provides no redundancy. Second you have a raidz1 pool with what appear to be >= 2 TB drives. Both vol1 & vol2 are at a relatively high risk of data loss. Search the forum for raidz1 and URE for explanation as to why raidz1 is not recommended for >= 1 TB drives. The parts of your setup that we know about make me curious as to whether your other hardware is up to the minimum specs for freenas.
Get some new drives, fix your underlying issue of not having enough storage or a properly configured storage system.
Whether or not you think pirateghost is being diplomatic, I would recommend you take his advice to heart. In general, the point of using freenas and/ZFS is to protect your data. In my view the potential data loss issues dwarf the issue of whether client machines have to map 2 shares. If data protection is not your primary concern, that's OK. But if that's the case, perhaps freenas is not right tool for the job.