Adding power components in frequency spectrum
Adding 5 dBm of noise would give you 5 dBm of noise. 

What I think you want to ask is, what value of noise, added to -65 dBm of translated jitter error, would sum to -60 dBm. The exact value depends on the noise AND JITTER statistics. 

If you assumed noise at -65dBm was completely decorrelated from -65 dBm jitter, then their sum would be 3dB greater, or -62dBm. (rms sum of the two amplitudes)

If you assumed complete correlation (highly unlikely!) then their sum would be 6dB greater or -59 dBm, failing to meet the spec.

Therefore the correct answer will be in the region of -65dBm (within a few dB) and I would use that as a reasonable starting point. Go ahead and compute the RMS difference between -60dBm and -65 dBm if you need a more accurate answer to get the last few dB if you need to.