You have not specified any parameters of the frequency-modulated signal, nor of the modulating signal.
If you already have a frequency synthesizer, you could modulate its frequency by applying the modulating signal to the control bits of the synthesizer. If you want to keep the synthesized carrier and apply a frequency modulation to it, then you can use a phase modulator.
Using a phase modulator will give you a narrow-band FM. If you need a wideband FM upon a fixed frequency, you will have to use a frequency translator method: you should build a lower-frequency frequency-modulated oscillator, then apply its output and synthesizer output to a mixer. The mixer output will contain combination bands, then using a band-pass filter you can select one band which you want.
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You can also add the modulating signal to Vo(t) in your schematic; this will achieve the frequency modulation, but you must test the system response with a real modulation voltage applied to VCO, so the feedback loop response is stable. I guess you can only achieve a narrow-band FM this way.