Can a Bitcoin ( or any other cryptocurrency ) be compromised with a large botnet/s?
Could a large botnet disrupt/hack the whole cryptocurrency system if
  the botnet currently holds more CPU power than the friendly nodes?


Yes, this is known as 51% attack. The malicious attacker should control >50 percent of hashing power in the network. 


  If yes,would it be feasible at all and or worth the work?


Current network hashing power is around 11 million TH/s. So the attacker should buy another 11 million TH/s. Bitcoin Antminer S9 has 14TH/s so you should buy around 800 000 of these. Let's say one is 2000$ so you have to invest 1.6 billion $. 

So it is relatively feasible. (Especially when the current bitcoin market capitalization is $136 billion)

Check also: Would any of the current botnets be able to launch and sustain a 51% attack?.

About is it worth it: this depends of what you consider by worth. It will probably make irreparable damage to BTC if 51% attack is successful, is that worth  investing 1.6 billion $. You can check if there is similar question of what 51% attacker could achieve (he can not gain 136$ billion of bitcoins) for more info.