To map an already existing category to a Jet category in magento 2, go to Jet > Jet Categories.
Now go to Catalog > Manage Categories and select the required category. Open the Jet Category Information tab. Select the Jet Category which has to be mapped to this Magento 2 category from the dropdown and then click on the “Save Category” button. When you map your Magento 2 category, all products of the mapped category get listed in Manage Product grid of Jet.