Found a temporary work around that SEEMS to be the IBM provided solution.
While viewing the database I want to extract FROM, I can type "XKEY 1000" and FileManager will spin through the databse and create a flat file contain just the primary key of every 1,000th root.
I can use that flat file as the key file in a keyed extract. It's the same thing I was going to do manually, but this saves me a few steps. the bad part is you still can't control the total number, plus you have to wait for it to scan through the ENTIRE database while it locks your session.