Appending json values to a existing spreadsheet

Is it possible to append the values of a JSON to a spreadsheet file where keys exist as column names?

[
   {
      "json":{
         "name":"emma",
         "user_name":"emma_123"
      }
   }
]

Yes, that is very simple. You just have to make sure that the keys and the and the columns have the same name. An example workflow which does exactly that can be found here:
https://n8n.io/workflows/29

Here also the video in which the workflow gets created:

While using google sheets it seems very easy, actually I wanted to append these values to a excel file, and spreadsheet node takes data in the binary form. I dont know how to convert it to binary data and map these values to the spreadsheet columns.

Then you can check out this workflow:
https://n8n.io/workflows/179

And if you have problems with reading the file additionally this one:
https://n8n.io/workflows/1

Ah and one more thing. To move data between “binary” and “json” you can use the “Move Binary Data” Node.
https://n8n.io/nodes/n8n-nodes-base.moveBinaryData