The first thing to understand is that there aren't any bitcoins in any wallets at all.
The only thing your wallet is storing is the private keys that allow you to create digital signatures to authorize the transfer of control of the value that is represented in the blockchain.
The blockchain is a public ledger that every full node on the network has a complete copy of. Transactions in that blockchain indicate how much value you can re-assign with signatures from your private keys.
Therefore, any change that you make on your offline computer alone will have exactly zero effect on how the value is distributed, since the value is actually represented on all the other computers on the network.
That being said, you could generate additional addresses (and when you do the wallet will generate a new private key for each new address) on your offline wallet. Then you could use an online computer to create unsigned transactions that will transfer the value to each of these new addresses in whatever values you like. Then you can transport those unsigned transactions (using whatever method you feel is secure) to your offline computer that contains the private keys. You can then use the Bitcoin Core wallet to sign the transactions on the offline wallet. You can then transport the signed transactions back to the online computer (using whatever method you feel is secure) to be broadcast to the network. Once the transactions are confirmed in the blockchain, the value will be under the control of the new private keys split up as specified by the transactions that you created.