You can't "add it to your plan" because it's WiFi Only.
If it doesn't have cellular, you can't use it as a cellular standalone device.
That said, if your phone supports it, you should be able to turn it into a Smartphone Mobile Hot Spot (SMHS), which creates a WiFi / Internet access point you can connect your iPad to
You need not add a WiFi only device to our T-Mobile account. T-Mobile does not provide a (widely available) WiFi network. You can share your phone's internet by turning on tethering (mobile hotspot) as smplyunprdctble suggested. Parks usually have Cable WiFi, if you are a cable subscriber, you should have your internet at most outdoor public places. I am a Cablevision subscriber and I use much more WiFi data on my phone that I do LTE data while I am outdoors.