I’m curious why you’d create a new iTunes account, but anyway…
If iTunes is telling you that it wants to erase what’s on the iPod, then it sounds like you’re one of those people who use the sync option for their iPod. You can only sync an iPod to one computer and if you connect it to any other computer, it will ask if you want to erase the contents and begin syncing to that computer.
You COULD say 'NO' to the erase prompt and then, while the iPod is still connected to iTunes, look at the options for that iPod and choose ‘Manually Manage Music.’
Then you can use that iTunes to add more music to the iPod.
As far as getting your music off that iPod and onto your computer, there are several ways and dozens of 3rd party applications that will help. Just search this forum or Google.