Fast turnaround: javascript that manipulates existing form

My situation is that I have an HTML form that I can’t alter directly. I need to (a) add some inputs to the form, and (b) when specific fields are filled out, I need the contents to be made into a json string and inserted into an existing field in the form. Again, nothing between the <form> tags in the sample HTML can be changed, it needs to be altered by js that loads after the form has loaded. The javascript needs to: 1. Insert a textarea field called “Comments” after DIV ID “bboxdonation_gift_fldAmountWithLevels” 2. Insert a field called “Print subscription number” after field ID “bboxdonation_billing_divPhone” (phone number field). 3. Insert a checkbox after DIV ID “bboxdonation_billing_fldAnonymous” that says “My mailing address is different from my billing address. When checked, it reveals the fieldset in #3 below. 4. Insert exactly the same contents as DIV class “BBFormAddress” to collect the mailing address: country, address, city, state, zip. 5. If the “my mailing address is different from my billing address” checkbox is checked: Take the contents of the inserted fields “comments,” “print subscription number,” and mailing address (#4 above) and create a json string. Insert this string into the field “User data merge” ID “bboxdonation_comment_txtComments” If the checkbox is not checked: Take the contents of the inserted fields “comments,” “print subscription number,” and the address fields in DIV ID “BBFormAddress” and create a json string. Insert this string into the field “User data merge” ID “bboxdonation_comment_txtComments” The json data inserted into the “User data merge” field should look like this: { "comments": "[contents of inserted field “comments”]", “printsub”: "[contents of inserted field “print subscription number”]", "address": [ {"country": “[country]“}, {"address": “[address]”}, {"city": "[city]"}, {"state": "[state]"}, {"zip": "[zip]"} ] } } See the sample form for the form code that needs to be altered by the js. You can use plain JavaScript of jQuery for this.