Several ways to do this. Here's how I do it, but it all depends on your parts.
1) add configuration in SW model. This could just be a block or a half machined part. A couple of examples for me are when I make motorcycle triple clamps. When the design work for the part is done I add a configuration called "stock", then extrude a simple chunk of material over the part which is the 'real stock' I order. I then suppress this extrusion in the default configuration.
I also make a couple of parts that has some turning work before the CNC work. I use this semi machined model as the 'stock' configuration.
In Solidcam, at the first stage you can select configurations (either default or stock) as your target and stock.
2) Let SC build your stock. Again at set up you select the part as a box (auto) or something like that. Select your target and SC will build material round it (a box) with extra material that you need.
3) As above, but select multiple parts for one big block.
Hope this helps.