Here's how I think we might be able to get Infected Bots..
When infected team is not 4 players, then make a fake player and join the infected team until there are 4 players. When they enter ghost mode (there is an event for that), and assuming fake players can event enter ghost mode (if they can't then just set up a timer).
Once they're in ghost mode, maybe we can force spawn them? z_spawn <whateverclasstheyare> and then give health should work. After that we kick the fake player out and the bot will take over, once the bot dies put the fake player back into the game.
Tweak the bot CVAR settings to make them better as an extra plus.
The only downside is that they wouldn't show up in the infected HUD.