You can separate a StaticMesh Asset into parts using Modeling Mode. Enable the Modeling Mode plugin, then use the “Select” Tool (in 4.26, renamed TriSel in 5.0) to select the triangles and the Separate command in that tool to turn the selected triangles into a new StaticMesh Asset.
There is a brief demo of this in a video here at around the 21min mark: UE4.26 Modeling Mode Intro - YouTube