REST Receiver XML to JSON. Force Array (Square Bracket) at Top Level

Dear Experts,

I have to consume a JSON format REST web service from third party. They expect to have a square bracket (array) at the top level of the JSON request message. Currently the format that is produced from the REST Comm.Channel is something like this:

{
   "FLD1":"VAL1",
   "FLD2":"VAL2",
   "FLD3":"VAL3"
}

But they actually expecting a format something like this:

[{
   "FLD1":"VAL1",
   "FLD2":"VAL2",
   "FLD3":"VAL3"
}]

How to force the square bracket at the top level with the SAP PI REST Comm.Channel?

Thank you,

Regards.