It might be...never tried myself thou.
I assume the player is axis with german units.
Designate the city as zone 1 (zoning in the scenario editor)
Place the unit the player might receive.
Edit the unit.
Add trigger: axis flag in zone 1 is equal to 1 (map condition)
turn [1,-x] (time condition)
if the scenario length is for example 20 turns and the player only has 5 turns that you have to deduct 20-5=15, so x=15.
If you need help, you can always send me pm in slitherine forum, same nick as i use here.
I am not so often on the matrix board, the slitherine one is more active.
Have fun :).
Author GTPG, Legacy of Versailles, The alternate, Panzer General original campaign for Panzer Corps.
Designer Operation Sea Lion for Panzer Corps.