I won't give you an equation to directly convert a dose rate to activity because it depends on the emitter and the receiver, i.e. the energy of the radiation and the interaction or the absorption in the material. A dose rate means how much energy is deposited in a mass per amount of time. The energy depends on the radionuclide, i.e. the type of radiation (alpha, beta, gamma) and the energy. However, there are tabulated values for almost all the known radionuclides where you can find the effective dose rate of a point source measured at a certain distance. The link I ll give you leads you to the radiation protection ordinance in Switzerland where in annex 3 you can find dose rate values for all the common nuclides. Column 6 (h10) gives you the effective dose measured in 1m distance of a source with an activity of 1 GBq. With this value you can estimate the activity of a nuclide by knowing its dose rate (admitted it can be considered as a point source).

https://www.admin.ch/opc/en/classified-compilation/19940157/index.html

Good @Thomas.

Thank you @thomas but if there is a direct relation a function of energy,  time and if possible shield thickness like the one exist in the radpro program