If the SDR is calibrated to report the signal power in dBm, you can convert dBm to an S-value using the customary 6dB/S-unit and S9 = 50microvolts (-73dBm) conventions.
It is quite common for amateurs to comment on their local noise level in S-units.
You must note that the total noise power detected by an SDR (or other device such as a radio, spectrum analyzer, etc) will depend on the bandwidth. As you increase bandwidth, more noise energy is integrated by the receiver. The power will increase as the square root of the bandwidth, assuming uncorrelated noise. Therefore it is important to note the bandwidth of the receiver used anytime you record a noise power value for comparison.
For an excellent representation of typical ambient noise levels by frequency of various types of location (residential, rural, quiet rural), refer to a chart published in the ARRL Handbook, and included in a talk by Rob Sherwood, NC0B at:
http://www.sherweng.com/ctu2014/NC0B-CU-2014-8c.pdf(see page 12). Note that this chart assumes a receiver bandwidth of 500Hz (typical CW case).
-Tony, K1KP