Is it possible to transfer the content in one PHP-Nuke into php-nuke of another site?

Are the prefixes the same?

yes

I don't see why not. Assuming you have the same modules/hacks/blocks etc. If you have users and the like on the second site that you want to transfer to, you would probably have to do it manually so you didn't lose their info. If you don't have anyone on there yet, and you are moving the site or something, then do a backup of your database, tell it that you want complete inserts, load that backup to other site.

If you just want to transfer part of the content of the site, and it doesn't affect user info you should be able to do it the same way, do a backup of that portion and load it.

The problem is if you have users or info you want to keep on the second site that you are loading the content to, you have to be careful that you don't loose your current info. Then I would do it manually.