Q: How do I transfer  songs from iPod with different iTunes account to my iTunes on my mac?

Redownload or transfer your iTunes Store purchases from an iPhone, iPad or iPod to a computer - https://support.apple.com/en-us/HT201267 - "This feature works only for content bought from the iTunes Store." As of OS9 no longer does apps which now must be re-downloaded directly from the Store.

Accounts cannot be combined. You can have items from multiple accounts in one library.