Skype is already encrypted:
https://support.skype.com/faq/FA145/What-type-of-encryption-is-used
Skype uses AES (Advanced Encryption Standard), also known as Rijndael, which is used by the US Government organizations to protect sensitive, information. Skype uses 256-bit encryption, which has a total of 1.1 x 10^77 possible keys, in order to actively encrypt the data in each Skype call or instant message. Skype uses 1024 bit RSA to negotiate symmetric AES keys. User public keys are certified by the Skype server at login using 1536 or 2048-bit RSA certificates.