Hi nedski ,
Thanks for your question.
I understand from your words that you created a PO by selecting several products in Stock Central and creating a PO with the "Add to PO" Bulk Action. Am I right?
If so, products can only be added in bulk to POs when creating one. Once created, you can only add products to a PO from the edit PO page.
However, there exists a workaround that may help you. You can create a new PO with the additional products you want to add to the original PO. Once created, just merge the two orders. You can find info about merging PO orders in this article.