You need a second JSON object, say tempJSON.
How you combine two jsons depends on their content.
For example, your main json contains books and the second json contains colors. Then you can do this:
-> tempJSON: Parse JSON string AJAX.LastData
-> booksJSON: Set "colors" to new object
-> booksJSON: Set "colors" to JSON tempJSON.GetAsCompactString("colors")
The last action is "Set JSON".
If both jsons are arrays, then you might need to iterate tempJSON and copy array elements one by one.