Can you put additional data in the payload?
The JSON that you see is only a nice way to look at a transaction, provided by the API you're using. The actual raw transaction, as it is transmitted on the network, is binary data that looks quite different.

The only place in the transaction that you can insert arbitrary data is the transaction script. This script is normally used to specify the address that the money can be paid to, but can contain arbitrary data. Note that if this data happens to be a program, it won't be executed by any (typical) client, it's just data.