1. You could allocate/configure that partition-space in Proxmox for Disk Image type, and then allocate a disk to the TrueNas VM using that space.
2. AFAIK - You could passthrough that partition (as is) to the TrueNas VM using the qm set command.
Researching this (option 2), even though a partition is not mentioned in the wiki, it appears in this post that a partition can also be used in the same way.