Yes you can thicken instant pudding mix.
When you make instant pudding and it turns out thinner than you would like it you can thicken it to make it to the thick consistency you want too.
The easy way to thicken instant pudding is to take some heavy whipping cream and mix it in with the instant pudding by adding a little bit of heavy whipping cream to the instant pudding at a time.
Do that until the instant pudding is thick enough for you.
It usually takes me 1 cup of heavy whipping cream to make the instant pudding thick enough for.
You can also add some powdered sugar and a cup of milk which also helps to increase the thickness of your instant pudding.
The whipping cream will usually be enough but the powdered sugar and milk also helps if you still want the instant pudding thicker.