Cygnas,
You need to insert the new assembly as a subasembly if you want to manipulat parts in the first assembly and make it flexible. I use the save asm as part for when I import neutral files that come in as asmblies, and was also usefull in old school wledments. It also works good for inserting supplied data into your asm that you will purchase such as a vavle it will reduce data size if you do not need all of the parts and pieces of the entire asm. Hope this helps.