You can delete bodies and surfaces from their respective folders at the top of the Feature manager tree. This will create a delete feature in the tree after the offset feature, and thereby leave the offset feature intact..
Yeah that worked great. I'm thinking now that I've finished the block; I probably should convert it to a solid, so that I can make threaded, & dowel holes,
for mounting. Needing to add fastener features, that step is what would make sense to me.
There are some great surface tools in solidworks.
There is offset surface, knit surface, trim surface and others. All of which should accomplish
what you are trying to do. What I would do is try one or all of the surface tutorials inside of Solidworks
to get your self aquainted with them