IDs are allocated by CS2D automatically. You can't do that manually and there is also no reason to do it manually.
Your script is bad if it requires a certain player to have a certain ID. Use variables!
Well,it's possible to make some bots,so the next one will use the required ID,then somehow delete the old bots. Really dirty and useless way. Why do you need it,anyway?
If you want to know the ID of the bot that you are going to add to the server. Just do a loop from 1 to 32, if there isn't a player existent there already, then that's the ID that the bot will take.
Ex.
1 2 3 4 5 6
for i=1,32 do
	if player(i,"exists")==false then
		botID=i
		break
	end
end