adding a column into the existing infotable through service

You will need to add the field to the DataShape itself by calling the AddFieldDefinition service on the DataShape entity. You might want to check if the field already exists before adding it, if you are planning to call this service multiple times

You may or may not need to add the field to the InfoTable, as in your example, depending on how the InfoTable is created (i.e. is it created from the DataShape? Did you add the field to the DataShape before creating the InfoTable? If so, it will already be there).

Here is an example:

//probably you want this to be at the end, so get a count of your fields on your InfoTable
//ordinals are zero indexed so the length should be okay, no need to increment it
var ordinal = Object.keys(infotable.dataShape.fields).length;
var fieldName = "FieldName";

if 
var params = {
	name: fieldName /* STRING */,
	description: undefined /* STRING */,
	type: "BOOLEAN" /* BASETYPENAME */,
	ordinal: ordinal /* INTEGER */,
	primaryKey: undefined /* BOOLEAN */,
	dataShape: undefined /* DATASHAPENAME -- only for InfoTable types */
};

// no return
DataShapes["DataShapeName"].AddFieldDefinition(params);

Generally, though, there is very little reason to try to dynamically append fields to a DataShape at Runtime, you should only have to create the DataShape once (which can be done programmatically, as in the above example), but you would want that to be a separate service than the one that is returning the InfoTable. What is the use case you are trying to solve?