I have a long history with trying to sync iPods with things they weren't meant to sync with. Back in 2005 - 2007, I used to sync an iPod shuffle and an iPod mini with Linux computers.
The problem is that iPods don't just recognise any music files you put on them. iPods have a 'music database' file that stores which files are on the iPod, and some metadata about the files (artist, album etc).
Apple don't tell other people what the database format is, because they don't like people syncing with other software. Any iPod syncing tool, other than iTunes, relies on people "working out" what the iTunes format is.
As time went on, Apple started hiding and even encrypting the iPod database. So most of the packages out there will work with the iPod classic, iPod mini, iPod shuffle, the first 2 or 3 generations of the iPod nano, and the 1st generation of the iPod touch (only if it's still running iOS 1).
I've seen packages like Senuti and iExplorer that can read files from iOS devices, but I haven't seen any other software that can add new music to them. Maybe Apple's hashing the database to make it incredibly difficult to add new things to, I don't know. When I moved to using Macs, I stopped needing to find backdoors into my own devices.
You have three options:
- Get a newer Mac, which you won't to because it costs money.
- Get an older iPod, which you won't do because it costs money.
- Jailbreak the iPod and find a music player that can see the iPod as a mass storage device. You won't be doing this if you're running iOS 6.1.3.
Short answer: Not going to happen.