How add home LaCie NAS to Finder sidebar on work iMac

First you'll need to make sure that the network volume you want to use is mounted first. Then you should be able to just navigate to whatever folder you're planning on using as your new home directory and select it. Network volumes in the OS X Finder can typically be found in the "Network" item under the main directory of your computer (in the Finder, you can hit ⇧⌘K to go there directly). The absolute path if you're using the terminal is typically /Volumes/Share Name.

It's worth noting that if you login without the network drive mounted, things could easily break. For this reason you'll want to make sure that you've set up OSX to mount the share on boot (prior to login). This doesn't happen with SMB (Windows file sharing) or AFP automatically, but there may be a way to hack it, maybe others can chime in with this part. NFS shares set up through Disk Utility mount automatically, but I'm not certain when in the boot process they do, I believe it's pre-login, but I can't find any information to corroborate this.

AFP mounts are tied to both a path and an authentication.

An exported volume might have multiple paths. For example, an OS X server might export its root, the /Users/ folder, and the /Groups/ folder. Each of these can be mounted separately. In your case, your NAS may be exporting its volume under multiple paths. It might even be exporting the same path multiple times. Check the NAS' settings.

Each AFP mount is tied to a particular authentication (username/password). You can mount the same volume/path multiple times using different authentications. Its possible that the credentials you used in those two different places are subtly different. Depending on the NAS, it might even come down to whether your long username or short username are used. Your NAS might not even require a username (only a password) and might ignore whatever username is given. OS X has no way of knowing that and so two different usernames (long versus short) might produce two different mounts even though they lead to the exact same volume.