[2019] Link Game to Uplay Account that already has the game on it?

Originally posted by SpectatorX: Yes, buy game on steam and then just log in to uplay with your existing account, that's all. The only "downside" is in uplay probably you will have two siege entries. All progress and items will stay as they are.

This! Once you launch the game from Steam it should link to Uplay, and you should see all your content from Steam. If you run into issues, reach out to the Support team so they can check on things. :)