Translations:Import from STL or OBJ/8/en

Conversion mesh to solid

 * Switch to part workbench
 * Make sure your mesh object is selected in the tree view, otherwise select it
 * Choose  →   from top menu
 * Specify tolerance for sewing shape (0,1 is default)
 * A new object will be created in the tree view (with blue shape icon, instead of green mesh icon)
 * Select the newly created object in the tree view
 * Choose  →   from the top menu
 * A new object will be created in the tree view and the previous one will be made invisible
 * Select the newly created object in the tree view
 * Choose  →   from the top menu
 * A new object will be created in the tree view, bearing "(Solid)" in its name, to indicate it is a solid

Since the created solid has no history and no editable features (like a simple copy in FreeCAD) you could delete all previous objects from the tree view. This would keep your file size small ...