One way, that bypasses the UI and therefore could be slightly risky, would be to ssh into the server as root (first enable root ssh under "system settings," "services," "SSH"), and then, at the command line, locate the pool and dataset, which should be somewhere under /mnt. Then enter the command to decompress the file, which I believe should be possible because Debian, which SCALE is built on, supports 7-zip. You'd just need to look up the command for decompressing files in Debian Linux, but it's easy to find it with a search.
A less risky way, which stays in the UI and doesn't involved the command line, would be to create a VM under the virtualization tab, assign it a network adapter (easiest would be its own physical adapter if you have an extra available, or else bridge it), and then create an SMB share of the dataset that contains the file you want to decompress. Then, inside the VM, access the share and decompress it. As for which VM, I'd suggest a Linux distro that supports 7-zip in the desktop GUI. I know that Mint does and suspect that many others do as well.