Yes you are more than welcome to ask and many of our hosts do go before or after eating.
We recommend writing in your request that you are interested in visiting a supermarket with your host. If you want to be sure, just choose a host profile with "Visit local supermarket" checked under "Characteristics."
Even if the host you want to meet does not have that checked, if you made sure to include that you would like to visit a supermarket in your request, mosts hosts will be more than happy to go as long as they have time on that particular date and time. Just don't hestitate to ask and most will be sure to find the time.