Unfortunately, First Free numbers cannot be disabled - the concept is embedded (at least in BAAN IV) in all the Order Type documents. You can, however, choose (at least in BAAN IV) between "No Series", "One Digit Series", or "Two Digit Series", taking an example from Purchase Order Parameters.
The other assumption is that your document number is numeric and sequential starting from some start number.
If you were lucky enough to have chosen "No Series" in your parameters, and you do not have any data yet in the system, then you just have to load the data, and then adjust the First Free Number to the maximum document number loaded.
It becomes more complicated if you have chosen other than "No Series".
It would be even more difficult to recommend a process where you already have data in the system.