I want to develop all-in-one add-in for Outlook. This means that this add-in will be available for all Outlook clients (desktop, web). I read here that i can do this. I already created add-in for Outlook Web Application and tried to deploy it to desktop client, but didn't figure how to make it. So how to make it set on desktop and outlook.com? Is that really possible? And if yes, how it can be done?
Technically you can't make an add-in that will work 100% on all clients. The web-based add-ins currently only work with Mailboxes hosted on Office 365 or Exchange 2013 or greater, and with Outlook 2013 or greater. Outlook.com integration is coming this spring. COM/desktop add-ins work with Outlook 2000-2016, but not in OWA.
To deploy an add-in you need to either publish it to the Office Store or an Exchange Server. See here for more info: https://msdn.microsoft.com/en-us/library/office/fp123515.aspx.
To install a self-published add-in that is not in the Office Store you just add the manifest file from the "Manage add-ins" page in OWA: https://msdn.microsoft.com/en-us/library/office/fp142256.aspx
Note also that you must host your web add-in code in your own web server, and secured with an SSL certificate.