You could always add a glass pane there with the windows but you also gotta make sure the zombies can cause damage and break the pieces, which once again will most likely be done through script.
i'm sure this can be done w/o external script
i'm going to play around with a window prefab and see what makes it tick if i figure it out i'll post my findings
:edit:
it seems from observation that the order that the zombies target the boards is controlled by script_noteworty and then a number
my hypothesis is if i make my window pane have a script_noteworty value numbered one more than the last board they will target it also would have to name it the same targetname as my boards also give it a spawnflag of 0 i assume but would have to figure out how to keep it broken it might just remain broken if i keep the dest value
this will only work if i add the window pane to my prefab window
i feel as though i've seen my next step in someone elses map does anyone know how to make a buildable window with the boards not originally on the window
my hypothesis is if the boards aren't built they'll target the window first
:edit: adding those values made them target the glass and it was still destroyable by shot/knife but makes it act the same as a board which makes sense
if anybody has an idea i'll gladly test it and let you know the effects
at the moment i'm not worried about the boards being placed on ground upon spawn only making the glass targetable by zombies to destroy without it being rebuildable
probably will have to make my own glass sections to be broken in order and pieces like five's instead of using the dest window model