Am I able to add a data-only sim-card to my dual-sim smartphone?
Data SIMs are no different from regular SIMs regarding data connection. As long as your phone supports the configuration you want (e.g. many phones only supports Slot1 4G + Slot2 2G configuration - confirm whether you still need 3G/4G on the call SIM and place in slot accordingly), you can just pop one in and enjoy.