The template is not open unless your open it.
You don't have to open it to extract the header from it, but you do to select something in it, so if you are going to use the selection object you must open it first, and as it was opened in the previous code, you can add the following before you close it
Code:
oTarget.Sections(1).Headers(wdHeaderFooterFirstPage).Range.FormattedText = _
oSource.Sections(1).Headers(wdHeaderFooterFirstPage).Range.FormattedText
If the document is created from the template, why doesn't it already have the information in the header and elsewhere?