Just back from a week on Mars*, so missed your post.

The reason why CH4 is being sought is that it is unstable wrt CO2, forming H2O + CO quite rapidly, even under Mars conditions. Therefore, if CH4 is found, some recent source is required. There are no signs of C-rich material on or near the surface of Mars so your hypothesis is unlikely, although a C-rich impactor could supply plenty, but this would be heat-processed in the presence of atmospheric CO2 and largely oxidised to CO + H2O.

* for those getting excited, only the 6th Mars conference at Cal Tech, but 2nd best thing to the real planet.