To Add a Block to a Custom Content Library (AutoCAD Mechanical Toolset)

In the drawing area, select the block to add to the content library.
Right-click and select Save as New Content.
In the Content panel of the Save Content As dialog box, select the folder you want to add the item to.
In the Save as section, specify a name for the new item.
In the View section, from the View list, select the name of the orthogonal view you are adding.
If the block is a sectional view, from the Type, list select the type of sectional view.
If you are not creating a sectional view, select None.
Click Save Content.