Is there any way to add commands that are executed in priviledged exec mode into the startup-config file?
For example I want to have the router start with the command terminal length 0
Why would you want to?
Commands in the config file are ‘privileged’.
Changing the config file takes a privileged user.
Short answer: no.
Specific answer: In this case, there’s a config command to do what you want:
conf t
line console
length 0
On some platforms you may need to enter a number after “console” above, IIRC.
This probably isn’t the best site for Cisco support – it would get inundated.