There is no way to add an estimated Training Stress Score (TSS)® values to more than one ride at once.
If you have a backlog of outdoor rides completed without power data (and thus, no TSS data), it's best to either estimate TSS for each ride individually, or leave the TSS blank for these rides.
Why?
We don't support adding "bulk" TSS data because we feel this wouldn't give riders an accurate enough picture of their historical TSS.
In order for our TSS estimation tool to be most helpful, rate of perceived exertion (RPE) information from the rider should be input for individual rides/events, and soon after the ride or event is completed. This way the effort level is fresh in the rider's mind, and they can input the most accurate RPE level.