The only way for you to invest in m-akiba from diaspora is to register your line in Kenya and then you can be buying bonds from abroad without having to travel to Kenya.
To buy m-akiba bond from diaspora use the pesalink option. Login to your online bank in Kenya, select PesaLink option, choose send money to phone option. Type 0733889889 as the phone number, enter amount of bond you wish to purchase. Under reason for payment/narration, enter your ID number, then submit.
What this means is that in one way or another, you have to travel to Kenya to register your line and open a bank account if you don't have one. Thereafter, you can be doing the deposits from your foreign country.