Any tricks to generate triangle wave to add to analog signal for oversampling?
It won't vary with input signal level the way you've shown it, however it will vary with impedance of the source. 

I suggest adding the noise with an op-amp to isolate the input from the noise source. You should probably have an anti-alias filter on the input signal before adding (unless it's naturally band limited) and make sure that the input is not correlated with the triangle wave.