No, because of security reasons you can't interact with external sites. This works with local forms:
---
File with the form.htm, to be loaded on the popup---
<html>
<head>
<title></title>
</head>
<body>
<form name="form">
<input name="field1">
</form>
</body>
</html>
---
Popup opener.htm---
<html>
<head>
<title></title>
<script type="text/javascript">
function openwindow(){
chill=window.open('File with the form.htm','','');
chill.document.form.field1.value='like this';
}
</script>
</head>
<body>
<input type="button" value="check this example" onclick="openwindow();">
</body>
</html>