Is this support alive?

You are asking a question in a forum that is manned by other Thunderbird users, so you would only get an answer if someone knows an answer.

One option is remotely taking over the 'Users' computer whilst User is logged in and running Thunderbird to add mail account as per normal - they would just have sit back whilst you do it, but they would see what you were doing.

If user has Thunderbird open then it would not be possible to edit any of the profile files. So creating a backup to use yourself would require them to exit Thunderbird to allow you access to the actual profile folders. The backup could be put somewhere where you have access to download eg: cloud.

If user has Thunderbird closed, providing the computer is still switched on, you could consider remote accessing of computer and creating a backup of the profile which you could then run on your computer - add mail account and then replace the backup. This means when user logs on and starts Thunderbird, the additional mail account will auto appear, so they would be aware that it had been created.