Normally additive jitter is not specified for the parts that are just fanout buffers with no PLL inside. Such a non zero delay buffer like CY2304NZ would not introduce too much additional jitter since they are just buffers (i.e. not that many transistors, simple circuit, etc.). Typical RMS Jitter that can be added to the output is approximately 10 ps for CY2304NZ and Period jitter will be less 10 ps for the CY2304NZ.
Attached are the measurements at different frequencies for a fair idea. In the document attached, kindly note that when you apply the signal from the used Agilent 8133A pulse generator (input), it has the jitter value as shown in Table 2. So Table 2 shows the jitter measured on the output of the Agilent 8133A pulse generator that is applied as the input to the CY2304NZ. The resulting output jitter is shown in Table 1 corresponding to the frequency and the type of jitter measured. So you can clearly observe that the jitter at the output is almost equal to the input. There is not much additive jitter from the buffer. So you need to have a clean source at the input.
