I did try this in an existing and saved assembly:
Dim oSwApp As SldWorks.SldWorks
Set oSwApp = Application.SldWorks
Dim oAssy As SldWorks.AssemblyDoc
Set oAssy = oSwApp.ActiveDoc
Dim oSwComp As SldWorks.Component2
Set oSwComp = oAssy.AddComponent4([an existing part], "", 0, 0, 0)
but i see no result.
SolidworksApi macros