I don't know of any way.

We aren't using that information right now, but I just went to play with the fields for a few minutes and they seem to be what they are.

I would think that entering, say, 8:00am-noon, while more time consuming, would be the same as saying "morning."

On the section where you can select the times a volunteer says they are available, can you change that to read, Morning, Afternoon or Evening as opposed to giving specific hours of a day?