How to add fields to this nested structure? Also How to use a condition for overwrite operation?

In the provided json add srcDo value as sos under postalAddress object in jolt

INPUT:

{
  "ewc":true,
  "PartyPostalAddress": {
    "item": [
      {
        "BlBdrCd": {
          "addrTyp": "N"
        },
        "LNCd": {
          "addrTyp": "N"
        },
        "PostalAddress": {
          "SrcAddrLn1": "1 KWN",
          "SrcAddrLn2": "Second Floor",
          "SrcAddrLn3": "1569 WQAS Blvd"
        }
      }
    ]
  },
  "SrcDo": "N"
}

EXPECTED:

{
  "PartyPostalAddress": {
    "item": [
      {
        "LNCd": {
          "addrTyp": "N"
        },
        "PostalAddress": {
          "sos":"N"
          "SrcAddrLn1": "1 KWN",
          "SrcAddrLn2": "Second Floor",
          "SrcAddrLn3": "1569 WQAS Blvd"
          
        }
      }
    ]
  }
}