yes, external designs (linked designs, sometimes call "XRef" designs - using an old AutoCAD term) cannot be edited/modified in the parent design (there is a new feature here, described below that helps). However, if you do Insert, followed by Break Link, will copy the features from the referenced design into the active design, so they can be fully edited, without affecting the original. I think this is probably what you are after.
The new feature is called "Edit in Place" - it is described here: Edit in Place . It is currently a Preview feature, but it works pretty well. This feature allows you to edit the external design in the context of the parent design, and reference geometry from the parent, as well. However, be aware that this does modify that external design, so all instances of that design will get those edits as well. If you want a fully independent copy, then use Break Link.
Jeff Strater
Engineering Director