How to add a controlled amount of jitter to a signal
One obvious answer is to use a digital signal generator to add a controlled amount of noise to the control input of a VCO.

Keep in mind that this noise signal will represent an instantaneous frequency error, rather than the phase error that you normally associate with jitter, so integrate/differentiate appropriately.

You show a separate circuit adding jitter to a clean signal coming from a test generator. The VCO could be part of a PLL in that separate circuit. The PLL will keep the average output frequency the same as the input frequency, but will have minimal effect on the added jitter as long as its feedback loop has minimal gain at the jitter frequency.

If you intend to generate more than a fraction of a unit interval of peak-to-peak jitter, you'll need an elastic store (FIFO) of some sort to hold the test data. It might be easier to just use the jittered clock to generate the data in the first place.