DSP: Superimposing quasi periodic signals

Hi everyone.

I'm new to signal processing. I'm doing a project in matlab that involves using the coherent averaging technique to remove the noise (generated digitally with a mean of zero) from biomedical signals (like egg and emg signals). To do this I need so superimpose the signals to cancel the noise but I don't know how to do it when the frequency varies over time. Is there some kind of algorithm to deal with this problem? If they were periodic I think it would be much easier. Thanks in advance for your help.

Regards,
John