Hi. Converters from z to t may return infinity if the value is too high, though this might be overcome using logarithms; z=8.72 would correspond to t=714 with 7 df.

However, I would be very cautious with such extremely high t-values. Are the values reported in the study really z-values? I strongly suggest to check it, as they could be t-values incorrectly labeled as z-values.