TrainingPeaks (including the free version), makes a TSS estimate for strength workouts with a total time and average heart rate
.
However, I agree with Alex on the limited accuracy of TSS related to strength training. Thirty minutes of HEAVY deadlifts and squats likely contribute to a higher level of central fatigue than, say an hour of arm & shoulder work. A semi-recent Fast Talk Labs podcast discussed the impact of lifting on cycling training. I believe they suggested rating workouts on an RPE scale, seeing how different RPEs affected your training, and then adjusting accordingly (e.g., scaling strength workouts to an appropriate RPE relative to your current cycling training/goals). Though I may be over-simplifying their recommendations.
That said, I still put my strength workouts into TP, but mostly as a reminder that the workouts happened (or some weeks, didn’t happen 
).