How to assign external email address abc@mycompany.com to abc@mycompany.onmicrosoft.com Sharepoint user

Hi

I need to setup alert to several users in the company (a1@mycompany.com, a2@mycompany.com) for Sharepoint Plan 1 users (a1@mycompany.onmicrosoft.com , a2@mycompany.onmicrosoft.com)

(1) how to add the a1@mycompany.com email address to a1@mycompany.onmicrosoft.com and 

                             a2@mycompany.com email address to a2@mycompany.onmicrosoft.com

if (1) is NOT allowed on SharePoint Online, HOW can I access the a1@mycompany.onmicrosoft.com email (do I need to subscript the email online service ) then I can login to a1@mycompany.onmicrosoft.com to setup the email rule to forward the email to a1@mycompany.com

Thanks