https://stackoverflow.com/q/63245317/2703456

gistfile1.txt

'genesis.json':
{
  "alloc": {
    "0xca843569e3427144cead5e4d5999a3d0ccf92b8e": {
      "balance": "1000000000000000000000000000"
    },
    "0x0fbdc686b912d7722dc86510934589e0aaf3b55a": {
      "balance": "1000000000000000000000000000"
    },
    "0x9186eb3d20cbd1f5f992a950d808c4495153abd5": {
      "balance": "1000000000000000000000000000"
    },
    "0x0638e1574728b6d862dd5d3a3e0942c3be47d996": {
      "balance": "1000000000000000000000000000"
    }
  }
}

Directly:

$ xidel -s genesis.json -e '{"alloc":{|{"0x397559475c2d6cf48e10d33b1ab399ccb5f4546b":{"balance":"1000000000000000000000000000"}},$json/alloc|}}'
# JSONiq syntax, or...
$ xidel -s genesis.json -e '{"alloc":map:put($json/alloc,"0x397559475c2d6cf48e10d33b1ab399ccb5f4546b",{"balance":"1000000000000000000000000000"})}'
# a mix of JSONiq- and XQuery 3.1 syntax (available as from xidel-0.9.9.6739), or...
$ xidel -s genesis.json -e 'map{"alloc":map:put($json/alloc,"0x397559475c2d6cf48e10d33b1ab399ccb5f4546b",map{"balance":"1000000000000000000000000000"})}'
# XQuery 3.1 syntax (available as from xidel-0.9.9.6739) all produce the same output...
{
  "alloc": {
    "0x397559475c2d6cf48e10d33b1ab399ccb5f4546b": {
      "balance": "1000000000000000000000000000"
    },
    "0xca843569e3427144cead5e4d5999a3d0ccf92b8e": {
      "balance": "1000000000000000000000000000"
    },
    "0x0fbdc686b912d7722dc86510934589e0aaf3b55a": {
      "balance": "1000000000000000000000000000"
    },
    "0x9186eb3d20cbd1f5f992a950d808c4495153abd5": {
      "balance": "1000000000000000000000000000"
    },
    "0x0638e1574728b6d862dd5d3a3e0942c3be47d996": {
      "balance": "1000000000000000000000000000"
    }
  }
}

------------------------------------------------------------------------------------------------

Variable:

$ var='0x397559475c2d6cf48e10d33b1ab399ccb5f4546b'

$ xidel -s genesis.json -e '{"alloc":map:put($json/alloc,"'$var'",{"balance":"1000000000000000000000000000"})}'
{
  "alloc": {
    "0x397559475c2d6cf48e10d33b1ab399ccb5f4546b": {
      "balance": "1000000000000000000000000000"
    },
    "0xca843569e3427144cead5e4d5999a3d0ccf92b8e": {
      "balance": "1000000000000000000000000000"
    },
    "0x0fbdc686b912d7722dc86510934589e0aaf3b55a": {
      "balance": "1000000000000000000000000000"
    },
    "0x9186eb3d20cbd1f5f992a950d808c4495153abd5": {
      "balance": "1000000000000000000000000000"
    },
    "0x0638e1574728b6d862dd5d3a3e0942c3be47d996": {
      "balance": "1000000000000000000000000000"
    }
  }
}

------------------------------------------------------------------------------------------------

Stdin, using Xidel's read()-function (http://www.benibela.de/documentation/internettools/xpath-functions.html#pxp-read):

$ xidel -s genesis.json -e '{"alloc":map:put($json/alloc,read(),{"balance":"1000000000000000000000000000"})}'
0x397559475c2d6cf48e10d33b1ab399ccb5f4546b   # Type or copy-paste this value.
{
  "alloc": {
    "0x397559475c2d6cf48e10d33b1ab399ccb5f4546b": {
      "balance": "1000000000000000000000000000"
    },
    "0xca843569e3427144cead5e4d5999a3d0ccf92b8e": {
      "balance": "1000000000000000000000000000"
    },
    "0x0fbdc686b912d7722dc86510934589e0aaf3b55a": {
      "balance": "1000000000000000000000000000"
    },
    "0x9186eb3d20cbd1f5f992a950d808c4495153abd5": {
      "balance": "1000000000000000000000000000"
    },
    "0x0638e1574728b6d862dd5d3a3e0942c3be47d996": {
      "balance": "1000000000000000000000000000"
    }
  }
}