Hi David,

I believe you can do that only using an API. You can use the plain java Client Libraries to do that.
Using the Plain Java Client Libraries you could use:
 
workItem.setCreationDate(timestamp);

I should probably create a better entry, but some hints on how to modify a work item can be found in this post: http://rsjazz.wordpress.com/2012/08/01/uploading-attachments-to-work-items/ .  You can do the modification in the operation execute instead of uploading the attachment. For example

@Override
protected void execute(WorkItemWorkingCopy workingCopy, IProgressMonitor monitor) throws TeamRepositoryException {
try {
IWorkItem workitem = workingCopy.getWorkItem();
workitem.setCreationDate(timestamp);
} catch (IOException e) { e.printStackTrace(); }
}

One option to do so would be to make the creation date attribute as editable from the process configuration source. (as it cannnot be done from the editor presentations directly.) The next step would be to synchronize all workitems so that the change is updated. Then one could go ahead and change the creation date of the synchronized workitems.

Hope it helps...