How do I add a new smtp server to an existing email account

I have an existing email account with an existing smtp server which only works when I am logged onto my home isp. When traveling I need to send emails but cannot do so because I am not logged onto my home isp. I have a different smtp server which works everywhere but I cannot install it as an alternate smtp server. How can I add an smtp server to an existing email account? or replace my home smtp server with a new one?