I would put some water in it and through in one of those toilet bowl cleaner disks that are made to go in the tank.

agree-put a bucket of water in it

First of all, if the sump pump is working correctly when you add water the pump will immediately pump it out. In addition, adding water is not only not the correct solution but it will in fact make the problem worse.

The sewer gas odor from the pit is a result of the decomposition of organic waste in the bottom of the pit. The organic waste may be microscopic in origin, carried in from ground water or water on the floor which then washes dust and debris into the pit. When the matter lays in the pit and gets wet microbiological action begins and the water becomes stagnant due to decay.

The solution is to clean the pit thoroughly then sanitize it with ordinary household laundry bleach. If there is any redisidual water laying in the pit you could then add a bit of bleach to prevent the bacterial growth.

Have you tested your house for radon? Radon does not stink, but one of the fixes is to seal off the sump hole and have a fan continously suck air from it. This may take care of two problems with one fix if radon is an issue.