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.