There are a lot of javascript tutorials out there, but the basic idea is this:
1) Name your forms (the original and the popup) by using the NAME attribute in the <FORM > tag.
2) In the popup form, use the "onClick" event to execute a javascript function that reads the checked values, puts those values into a string variable, then populates the appropriate field of the original form:
<b>myForm1.toField.value=stringOfData</b>
and closes the popup window.
This being a PHP forum, I don't want to get too much into the javascript code here. If you need more help with that, email me and I will give some examples.
-- Rich