direi:

codice:
<form name="formalto" method="post" action="">
    <input name="nomepreso" type="hidden" id="nomepreso" value="xxx">

<input type="button" onclick="this.form.nomepreso.value=parent.alto.window.document.formalto.nome.value;" value="cliccami tutto" name="b" />
<input type="submit" name="Submit" value="Submit">  
</form>
ciao