Before I bought a compatible HR strap I, on occasion, would import Garmin and SRM data into GC and then export to CSV and edit the data with excel. Generally it just involved taking a column of HR data from the garmin file and replacing the HR column in the SRM file. Once complete you can re-import back into GC.
I've also plotted data in excel from a wired powertap recording at 1.26Secs/pt and compared it with power from an SRM recording at 1Hz.