Hi Prince,
Yes, you are right, this is not a current feature of PresentationBuilder. Some amount of work would need to be done in order to support this. Currently, this work is not on the schedule, though.
One possible way to approach this issue – use PresentationBuilder as necessary to assemble presentations that do not contain WebExtensionPart. Then use System.IO.Packaging to add the WebExtensionPart to your presentation. There are no restrictions for adding / deleting parts when using System.IO.Packaging directly.
-Eric
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