Multiple transfer rate

Im kinda new to OpenMC, and currently im currently doing small research on MSR reactor where there is continuous fuel processing. In OpenMC there is trasnfer rate feature, so i can get rid all the burnable poison like Xe135, I135, Gd157 and so on. What i wanted to do is not just in transfering all the disadvantageous poison, i would like tto add transfer rate on the MSR fuel so it can continuously running.

The problem is when i use the integrator.add_transfer_rate, it takes the latest command only so the previous on getting rid the poison isnt intact to the whole code. How to solve this problem, how to add more than 1 transfer rate feature.

Thank you in advance.