In a LAN host add your custom bots when you are spectating before joining. That will override the configs\mapname.bots file. The mapname.bots file is loaded automatically when you join an empty game or when you leave the game to spectate making it empty.
To add your bots write a config (.cfg) file
Code:
addbot mybotname
addbot mynextbotname
...
as described in the
wiki , save it in \fritzbot directory and /exec it in the console before joining the match.
And remember if /sv_pure is one then all files including your .cfg have to be in pk3 files, so set /sv_pure 0 to make it easy for yourself.