You can get all full form collection; not only this you can also read the value of global variable of parent page in to newly opened window. Let's see how.
I created one variable into parent page. And somewhere I call window.open() to open new page into new window.
gblPageTitle = 'Welcome to ';
In parent page I just have to write following line to get that variable.
document.title = 'Questions on '+ window.opener.gblPageTitle;
And now lets get the full document of parent page or specifically form collection.
var previousForm = window.opener.document.forms;
Have fun with window.opener :)