I need to create only external surface of an assembly. It is possible by creating copy geometry and selecting each and every surface. It takes a lot of time and I don
Try a shrinkwrap - this only uses the external surfaces, and saves it a a part file.
It's under File -> Save a Copy -> shrinkwrap (in drop-down box) - you may need to play around with the detail level (from 1-10) to get the results you want.
I don't ~think~ (?)there is a way to. It might help, tho', if you'll familiarize yourself with seed and boundary, etc. selection techniques. Solid Surf copies, then deleting void lump quilts, silhouette trims, etc.may help, too. If you need to turn all of it into Solid objects, you've probably got some work to do.
You could try merging all the parts of your assembly into a new default placed dummy part. Then, you could simply copy solid surfaces of components in your assembly into this dummy part and hide solid geometry if you needed to do so. Depends on how may parts are in your assembly really but bear in mind you would only need to merge those parts visible to the eye i.e. the exterior geometry. I've used this method a few times when I have been unable to use Shrinkwrap. Using this methos would also automatically update geometry in the dummy part (provided the assembly was in session) but would not automatically merge any new parts into your dummy. It's one workaround option for you to try?