I'm not sure exactly what you mean. Brushes created in Photoshop 5.0 will work in Photoshop 6.0 as well as 7.0
They just aren't backwards compatible. If you wanted to save the brush set to only work in 7.0 my best suggestion is to make jpeg templates of all the brushes and then create the set from the jpegs in Photoshop 7.0