How to insert background EM noise into pathloss equation?
The question is this: Could I consider this atmospheric noise as a fundamental minimum in dB that a signal has to have in order to be able to be detected by an antenna.


If any of the desired signal is present within the capture area of the antenna, the signal is received regardless of the noise that is present. That is to say that the noise is simply another signal that the antenna receives (with the same gain) in addition to the desired signal. The receive antenna system will also generate a limited amount of thermal noise.

The Friis formula allows you to calculate, for a given frequency and antenna system, the ratio of received power to transmitted power. That is its primary function.

The question of what is the minimum signal to noise ratio to carry out communications requires additional calculations. It is dependent upon the receiver, modulation method, the receive bandwidth, and signalling rate. There are communication methods that allow a -20 dB signal to noise ratio at the receiver and communications can still be carried out reliably, for example.

Your posting has several issues that you need to address. A dB is always an expression of a power ratio. When you say:


  so would this mean that the receiver antenna has to receive higher than 294.15 dBm signal


the m following the dB means you are referencing this to a milliwatt. The absolute power level you are quoting is 2.5 x 1026 watts. Not likely.

In other cases you use the term dB with no normative reference which makes your statements ambiguous. The proper use of dB is a fundamental requirement to communicate effectively in this discipline.

Then there is this:


  A 1 Hz magnetic signal, going between 2 magnetic antennas


There is no such thing as a magnetic (only) signal nor is there a magnetic (only) antenna. In RF and antenna engineering the signals are all electromagnetic (EM) signals. They always have an E and H field. There are antennas called magnetic antennas but this is simply a classification of a loop antenna that is still a conventional EM antenna.

I have seen from your past posting that you tend to apply boundary sphere conditions and you have done it here again:


  with a boundary sphere of 20 cm has a path loss of roughly 16 dB


I encourage you to drop this line of thinking if you are interested in antenna engineering - it is sending down an errant path. The proper reference is the dBi gain of an antenna, not its boundary sphere.