You can only convert airtime bought from KCB account by mistake to mpesa if the transaction was not successful. If the transaction was successful, then there is no way you can reverse it.
The only option you have is to use third-party businesses that convert airtime to Mpesa.
However, most of those businesses will only give you 70% back. For example, if you send them airtime worth Kenya shillings 1,000 there will give Ksh. 700.