External method code:

def get_next_id(ZClass, Folder=None):
    """Generate a unique sequencial id for the ZClass"""
    ZClass.next_id = getattr(ZClass, 'next_id', 0) + 1
    id = ZClass.getId() + '_' + ZClass.next_id
    if Folder and id in Folder.objectIds():
        id = get_next_id(ZClass, Folder)
    return id

Place the above code in a ".py" file in your Zope extensions 
directory. Then create an external method "get_next_id" 
inside Zope in the Root Folder (!) that exposes the method 
function. Change the following line in the constructor 
DTML method of your ZClass (ZClass_add):

<dtml-with "ZClass.createInObjectManager(REQUEST['id'], REQUEST)">

to:

<dtml-with "ZClass.createInObjectManager(get_next_id(ZClass, this()), REQUEST)">

Substitute the actual id of your ZClass for "ZClass" in the above line.