Mount network volume as local drive for data recovery
Even if you could do this, the network indirection means the tool cannot read raw data and will almost surely not yield the results you would hope for. The data that was deleted exists only on unallocated blocks - not exposed through the NAS device and then sent over a network protocol showing the live state of the filesystem after the rm had done it's damage.

If you're serious about the data recovery, you'll stop using that volume (disconnect all clients) and call a professional data recovery firm that knows how to reconstruct various RAID situations by imaging each drive directly and then reconstructing things. A call or support ticket to the storage vendor is usual, but in this case, I don't see any "file recovery" info on the FreeNAS web site.

Knowing exactly the version of NAS controller will help you get a good quote on how much the recovery effort will cost. In most cases, their tools and knowledge coupled with drives that don't have failures and need a clean room resuscitation will reduce the cost of recovery to the time and tools needed for an expert to reverse the file deletion.

They will also be able to query you to see if you perhaps have overwritten all the data and perhaps want a quote before you commit to recovery.