Assuming the requirement is to run a manufactured part through the purchasing process, you can change the Part Type of the inventory part to “Purchased” and save. This will create a product structure header with type “Purchase” and a purchase part. 

If the main sourcing method of the part is to manufacture it, change the Part Type back to “Manufactured” (this will not remove the purchase part, thus it will still allow purchasing of the item).

As explained by @ChamilaW  it is fine to both manufacture and purchase the same part in IFS. The part type can be seen as a default.

Important to remember however is to use a cost template for the part that captures both manufactured and purchase costs. For the part cost calculation you have an option on the cost template to use either manufactured or externally acquired (purchase) costs

Still, you have to notice that important accounting parameters such as: Accounting Group, Product Code, Part Product Family are defined for the Inventory Part. So there is no way to have accounting rules and accountings different for one Inventory Part that is mainly manufactured but exceptionally purchased. This is quite crucial in e.g. Polish law, mostly ends with separate Inventory Parts.

BR, Lukasz