Can transaction data be altered before creating a block?
You can modify it, but in doing so, you will invalidate the signature on the transaction, since the data is no longer the same as what was signed.

You could do so on transactions that don't involve signatures, but they are generally non-standard on the mainnet.

Note that even after modification, you can only update it within the bounds of the input(s) sum. You can never modify a transaction to have an output greater than the sum of the inputs.