I made a modification of this plugin. Now, you can take back grenades from bots by shoving them!
Also, two new cvars:
l4d_pass_grenade_bot <0/1> "Allow giving grenades to bots? 1 Enable, 0 Disable" (Default: 1)
l4d_take_grenade_bot <0/1> "Allow taking grenades from bots by shoving them? 1 Enable, 0 Disable" (Default: 1)
Note: You need to use a primary or secondary weapon to shove the bot with, and not carry a grenade yourself, for this to work.
Otherwise, you can use
[L4D & L4D2] Gear Transfer, but it is a bit bloated.