With the help of iJoobi I was able to find a much easier way to do this with the free version of Acajoom. In your form make sure there is a firstname field with the name First_Name_input and a surname field with the name Last_Name_input and an email field with the name Email_Address_input then in the submit pieces for the form put in a custom end submit as follows

// load the standard library
$this->execPieceByName('ff_InitLib');
$name = ff_getSubmit('First_Name_input').' '.ff_getSubmit('Last_Name_input');
$email = ff_getSubmit('Email_Address_input');
header( 'Location: index.php?option=com_acajoom&act=subscribe&listid=14&receive_html=1&email='.$email.'&name='.$name );

All this does is after the form is submitted it forwards through and passes the relevant values to the Acajoom subscription form. It means the uyser will see the Acajoom message after the form is submitted so I just changed administrator/components/com_acajoom/language/english.php to have a generic message that thanks the user and provides a link back to the homepage so that it would be a suitable landing page for any form submission used on the site.

I'm sorry I'm trying to do the same thing.
I could explain in detail how to do?

are Italian and use a translator