You need to write editor plugin that creates static mesh actors within current level.
Here’s related question on answerhub:
https://answers.unrealengine.com/questions/246723/addactor-in-the-editor-via-c-module.html
Basically, GEditor->GetEditorWorldContext().World()->SpawnActor… (op in the question used AddActor, but spawn works just fine).
Followed by actor setup (static mesh component, etc), followed by
AStaticMeshActor->MarkComponentsRenderStateDirty();