Set your profile for the equipment that you have; namely for ten gallons. However, indicate in the recipe section that you are doing five gallon batches.
In your equipment profile you should include the dead space of your mash tun and your boil kettle. These figures will be used to calculate how much liquid to start out with to end with five gallons in the fermenter.
This should take care of everything.
I went home last night to look at the program to see what equipment profiles are already in BeerSmith. I noticed that there is one profile for Stainless Kegs. While this is not totally what you need, it is a start. I would copy that equipment profile, and then modify the copy for your exact setup.
For instance, I don't know if you will be using the keg for a mash tun or the kettle. You could decide which one to use for the mash tun, and then accept the default settings for that item. You could then modify the other settings for the equipment that you have that are not currently in that profile.
I think that this would get you to where you want to go.