Found it:


private static BuildingBlock GetQuickPartFromTemplate(Microsoft.Office.Interop. Word.Application wordApplication, string wordTemplateDocumentPath, object quickPartName)
{
return (from Template template in wordApplication.Templates
where template.FullName.Contains(Path.GetFileNameWithout Extension(wordTemplateDocumentPath))
let index = quickPartName
select template.BuildingBlockEntries.Item(ref quickPartName)).FirstOrDefault();
}

https://stackoverflow.com/questions/...s-using-c-shar

You can call it like:
string temp = Environment.GetFolderPath(Environment.SpecialFolde r.ApplicationData) + @"\Microsoft\Templates";
BuildingBlock b = GetQuickPartFromTemplate(doc.Application, temp + "\\Normal.dotm", "ThePartIJustCreated");

Range rng = doc.Application.Selection.Range;
rng = b.Insert(rng);