Fit Rice Distribution for negative RSS
You know how to map from dBm to mW and the other way around so it seems to me this is a lossless transformation and you should be good to go by converting the dBm value to the mW value and doing the distribution fitting. You can then use it for whatever you want but just remember that you have done the conversion so will need to convert back to dB scale if that is what you want to report in the end.

Edit: 
From the comment, since MATLAB is complaining about too small of values I suggest just converting to microwatts. So do:

z = 1000*z;
pd = fitdist(z, 'Rician');


If it still complained, we could just keep moving down to nanowatt and so on. As long as you know that when you use that distribution you need to keep the units straight, then there is no harm in going from dBm to mW, $\mu$W, nW, and so on.