They are there already.
If you set up a defensive scenario, you have the options, Dug-in, Fortified and Dug-in + Fortified.
Designers may have a different concept, but I se these as:
6. It takes 1 hour for a unit to deploy for defence or go under cover (e.g. into harbour area) – a unit passing through cover (on road or cross-country) may “halt under cover” with no time delay.
7. It takes 5 hours for an Inf unit to deploy and dig-in for defence. A unit can take over a “dug-in position” from a friendly unit or re-occupy in 1 hour, or an enemy position in 3 hours.
8. The 5 hours of digging by a unit gives it slit trenches with thermal screens. If a unit remains in defence for 24 hours it is assumed that it has improved its defences to include 18” overhead cover, some mines and some wire for local protection. This improved defence gives it increased probability of success when attacked, (see Appx. “A”). A unit taking over a 24 hour position from a friendly unit occupies it in 1 hour but from an opposing unit will still require 24 hours to claim full protection."
Curry, John. BAOR Operational Wargaming 1950-1960 : The British Army Tactical Wargame (1956) (p. 1). The History of Wargaming Project. Kindle Edition.
i.e. Dug-in represents 5 hours of digging-in, and Dug-in + Fortified represents 24 hours.
On the other hand, if you are looking for a more comprehensive, in depth prepared defense, that is not mobile warfare and AB is a game simulating mobile warfare.