Can I use an iPod nano 8g as a flash drive for windows, and still use it to play the music I paste in that drive?
No.
Apple does not want you to manage your own music.
Apple specifically engineered their products to prevent you from easily dragging and dropping files for quick portable usage.
Reasoning behind this is to encourages more sales from iTunes by forcing users to only use iTunes to manage music.
To quote Apple,

When you use your iPod as a hard disk, you can't see the media iTunes
copies to your iPod in the Finder or My Computer. This is normal.
Using the Finder or My Computer, you can't copy these media files from
your iPod back to iTunes or to any other computer. The synchronization
between your iPod and your computer is one way: from iTunes to iPod.

https://support.apple.com/en-us/HT201567
A warning: Keep your backups handy because once your music is in the iPod it's difficult or impossible to get back