That's a great idea .... there was a year I would have paid you a million dollars if you could have reminded me BEFORE I made that mistake! You can utilize those extra email fields by sending a ticket to Support and asking them to turn on "Custom Objects" for your account. Custom Objects are a powerful feature of ONTRAPORT that is available on request.
Don't be scared by the complex nature of Custom Objects (it is really designed for database gurus). In this case, you just need to have Custom Objects enabled for your account and you can start using that extra email field in Sequences, Rules, etc. After it is enabled for your account you will see all of the email fields you have added in the Send email to drop down:
Note that we still enforce our Rules regarding opt outs, bounces, etc. If the recipient of the email clicks the link to "Unsubscribe" from bulk email the Contact Record itself is set to "Opt Out", preventing bulk email being sent to any email address in that Contact Record.