Skill levels are defined by two variables; skill and aim skill.
Skill is used to specify the bots map navigation ability and is in a range from 1 to 3 (3 being the best)
Aim Skill is ranged from 1 to 4 (4 being the best)
These are either set globally in a server setting with /bot_skill x and /bot_aimskill x or you can specify each individual bots skill in the bot config file:
addbot <botname> <skill> <aimskill>
Here's an example of adding an axis engineer with the highest level:
addbot gondi 3 4