Irule to insert source IP in UDP payload

when CLIENT_ACCEPTED {
  set payloadLength [UDP::payload length]
  set address [IP::client_addr]
  set addressLength [ string length $address ]
  UDP::payload replace $payloadLength $addressLength $address
}

You can also use the CLIENT_DATA and SERVER_DATA events and take a look at https://clouddocs.f5.com/api/irules/UDP__payload.html