I can see that it got taken to hackers and they got in it
The "hackers" could not possibly "get in" unless they knew your correct Passcode, Apple ID and Password.
Hopefully, you did not respond to a scam message from "Apple", "Apple Support" informing you that your phone has been found and you just need to provide more information so the phone can be returned to you.
Apple does not get involved with lost or stolen phones, so you know that any message like this is a scam that is trying to trick you into providing personal information so the bad guys can access the data on your phone.
can I add an eSIM to it so it will be online?
It would be impossible to add an eSIM to a stolen phone that does not have an Internet connection. That is the reason why the phone is "Offline". It does not have an Internet connection. If the phone had an Internet connection, you would not need to add an eSIM.
So, why does the phone not have an Internet connection? There are several possible reasons for this:
- The battery has run down
- The phone was powered off and there are no other iPhones in very close proximity to pick up the intermittent signal that your phone is sending out.....even though it has been powered off.
- The bad buys have wrapped the phone in aluminum foil or placed it in a Faraday bag so the phone cannot send or receive a signal at all.....even though the battery might be OK and phone is powered on.
Unfortunately, all that you can do is keep checking using Find My a few times each day to see if the status of the phone has changed.
The bottom line......although you will never see the phone again, your data is safe as long as you were using a Passcode on your phone. There is no way that anyone could ever guess the correct Passcode in less than 10 tries before the phone locks out and won't even allow any more Passcode attempts.