Yes it is possible.
I did the following myself:
0. Pause all torrents on Transmission GUI
1. Disable Transmission using the Zyxel admin page (the same place you installed Transmission in the first place)
2. You need to log into your NAS with telnet or SSH ( you can enable them in NAS Control Panel => Network => Terminal )
3. You need to edit the config file directly in the Transmission directory (using vi for example... google for vi usage guides... https://www.cs.colostate.edu/helpdocs/vi.html
seems pretty OK.).
On my NAS542 (firmware V5.11(ABAG.3)) its at below location:/i-data/sysvol/.PKG/Transmission/config/Transmission/settings.json
You want to change ""download-dir"" and maybe also ""incomplete-dir"" to be your desired location... ie mine looks like:"download-dir": "/etc/zyxel/storage/sysvol/toshiba/Transmission/download",
- "toshiba" is then my share name for my toshiba drive.
4. Then Enable transmission using the Zyxel admin page so it reads the new config.
5. Resume/start previous torrents and add new ones. Already existing torrents will probably remain in old locations though, the default dir is for newly added torrents.
credits: used Winterwolf's guidance as base ( viewtopic.php?f=249&t=10055