join, merge, combine surfaces with solids??

Sorry, I'm not familiar with mold making, however I believe what you want to do is use "replace face" to replace the old solid faces with the new surface.

Also, you can use "delete face" to delete all the faces you do not want (thus making the solid body a surface body), then use the knit tool to knit all the surfaces together. Just make sure to select "convert to solid" in the knit properties.

If you're making shut off surfaces manually, you need to arrange surface bodies in the correct folders at the top of the feature manager. You might want to look into the help or training documentation about using mold tools with the cavity and core body folders.