We only accept domain transfers on a one year basis. That is why you are unable to add more than one year to a domain transfer at the time of checkout.
However, you can wait for the transfer process to complete. Once the domain can be seen and managed in your Cosmotown account, you can renew it for multiple years.
To Renew your domain name you may follow this guide:
Please note that domains are only allowed to be registered for a total of 10 years at any given time. So, if you have 2 years left on your existing registration, you will only be allowed to renew for 8 years or less.