Because phase noise is actually a power level at a specific distance from the carrier, combining two signals, the phase noise result will be a summ of both phase noise levels.
For example, combining two signals having both the absolute value of the phase noise of -100dBm/Hz at 10kHz from their carriers, will result a signal having an absolute phase noise level of -97dBm/Hz at 10kHz from the carrier (3dB higher, or double the power).
The LO phase noise (or the PLL) is the greatest contributor to the phase noise of a system.
For example phase noise added by a diode mixer or by an IF amplifier is negligible compared to a PLL or LO phase noise. Here, doing the analysis, don't have to confuse phase noise with noise figure.
If the RF input is strong, its phase noise may be also a contributor to the receiver system phase noise. This is the reason when doing receiver blocking tests (strong interfere at RX input), the signal generator used for blocking signal should have very low phase noise, usually at least 10dB lower than the receiver system phase noise.