Or you can retrieve their values, then post them to the server via an XMLHttpRequest.
That would not be attached "to the posted form", it would be a
separate request and likely to fail in a greater number of browsers
(though how much greater I'm not going to guess
).
If the OP's intention is to send the data without posting a form, then
script can be used to append the data to the search string of the URI
for an image, script file, hidden iFrame or page, or any other request
to the server - there is no need to rely on browser support for
XMLHttpRequest.
However, this is not at all appropriate.
ACK.