Yes. You can transfer NP from your main to your side accounts to be used for gallery upgrades, checking your pets into the Neolodge or for storage purposes. You are NOT allowed to receive NP from other players to your side account. You are allowed to open a bank account on your side to store Neopoints but you cannot collect interest.
If you've reached the maximum Neopoints your bank account can hold on your main, remember that it is against the rules to receive payments or Neopoints directly to a side account. All Neopoint transactions must be completed on one account—your main account. This means: If you sell an item, the payment must be received on the same main account. All items sold must be sent from your main account whether it is via trading post, item transfer, auctions or shop. After receiving Neopoints on your main account, you can transfer them to a side account for storage.
It is recommended to highlight your main and side accounts in your 'Name' field.
To be recognized as belonging to the same player, your main and side accounts should all be registered under a single email address.