Why Service Titan do you have to my my life hell? I should be able to add an estimate to a job made from that job.
At anytime you can add estimates to a job. Make sure you have the correct permissions to do so.
And if you are talking ST mobile, as long as their is one estimate on the job, you should be able to find it in Mobile Followups and edit that estimate or create new estimates for that customer on the same opportunity.