How convert radiating intensity to transmit power of cellular base station?

For research I'm doing, it is possible to convert a cellular relay base station's radiation power intensity to a transmitted power from this cellular station?

Not sure which quantity you designate "radiation power intensity". The usually observed quantity is field strength (or incident power density).

In any case you need to know the transmitter antenna gain respectively directivity. GSM uses antenna arrays with high vertical directivity.

Generally refer to free-space path loss equation.

OK.
What is the suppose output power (worst case scenario) at cellular base station ?
I have only the radiation intensity of each cellular base stations.

Intensity is measured in Watts/meter^2
power is measured in watts

for a point source radiating some power P, the appropriate "meter^2" is the
surface area of a sphere of radius R, or 4 pi R^2

thus the intensity drops off as 1/R^2

most sources can be approximated as a point source
one exception is the antenna, which has a tendency toward directionality

you can make a first order approximation assuming a point source
beyond that, you'll need to have an approximation of the radiating pattern of your antenna