Dear Lin,
as you have pointed out, and as it is said in other post like in Link, FLUKA gives you the possibility of scoring the isotope production yield in a fixed geometry. Therefore, I fear there is no solution for your case. The dynamics coming from the flowing of the liquid lead must be introduce by yourself separately.
I agree that the best way to score the isotopes yield in the lead region is with RESNUCLEi card (without associating a DCYSCORE). To the results coming from this detector (nuclei/primary), you will need then to apply the dynamics of your specific case.
Best,
Giuseppe