There are many ways how you can integrate downloadable content into a Spine skeleton.
You could create downloadable atlases which group multiple cards (or just a single one) to a download-bundle, and then load the required attachments from this separate atlas similar as in the example scene
Spine Examples/Other Examples/AtlasRegionAttacher.
You could also prepare each card as a Sprite asset, then download only the required Sprite assets and assign them as attachments as shown in the
Spine Examples/Other Examples/Mix and Match and
Mix and Match Equip example scenes.