How to assign activity in mCi/MBq for nuclear medicine applications?

I am currently working on a project where we want to estimate absorbed dose to an organ from different radionuclides e.g. I-131, Y-90, Po-210, etc. Is there any way to assign source activity in mCi or MBq in Geant4? The header file SystemsofUnits.h has Ci and Bq defined in it, but I have been unable to find a way to assign activity in those units to, for example, a GPS source. At the moment, I am simply simulating, for example,10^6 particles if I want to simulate a source with an activity of 1 MBq.