How to do I keep music on an iPod when I have no access to the iTunes account that it was added with?
You could try, the windows only application, MediaMonkey (mediamonkey.com). This may help you as it used to be able to get around the itunes media controls... I have not used it for a while, so it may have changed (or Apple may have changed something to stop MediaMonkey from working as it used to).

With MediaMonkey you may be able to save the music already on your ipod. Once you have saved the music (safely - check it first) to a PC, then sync to itunes and then after you have sync'd it with itunes (which is pretty much guaranteed to wipe the original music), you could then re-add it...