With the input terminated in 50 Ohms, external attenuators do not generate more noise as they are at the same ambient temperature.
With a good spectrum analyzer you can reduce the noise floor by adjusting RF badwidth, video bandwidth and sweep rate, more than by input terminations.
If not satisfied, you should use a preamplifier for a selected frequency range. Spectrum analyzers use input harmonic mixers which have higher noise figure than a good preamplifier.