Combine two signals of different frequencies in a test setup
Could you make a little transformer, send the main signal through it and couple your high frequency signal in?  (I don't know where to get the transformer core that will work, but perhaps you do.)  I should add that I've used little caps (a few pF solder on, or a few turns of insulated wire wrapped around a lead.) to inject signals into higher impedance circuits.  I didn't think that would work here.