Like other types of Products, Timeblocks can be assigned other Products as Addons, and Timeblocks can be Addons for other Products. The Schedule Builder has an Addons screen, and it is displayed to your Customers if there is an Addon Product assigned to the selected Timeblock.
There are no System level configurations related to this feature.
Timeblock Products can have Addon Products be either Required or Dependent -- Suggested Addons are not supported. Create your Products, and then assign them to a Category/Region (preferred) or directly to the Timeblock Product.
Below, I am assigning a Merchandise Product with Variations as a Required Product to a Category. It is my 2017 Camp Uniform product, and I am requiring everyone participating in something from the Category purchase the uniform. I logged into the Administrative side of my database, then navigated to Settings --> Setup --> Configuration. I selected my Category, found Required Products, searched for my uniform Product, then dragged the Master from the Unassigned to the Assigned column. 
Below, I am assigning a Timeblock Product as a Dependent Product to another Timeblock Product. It is my "PM Aftercare" product. The aftercare is only available for one of my Products, so I am only assigning it to that specific Product. I navigated to Products, found and selected my Product that has aftercare available, selected the Assignments tab, found Dependent Products, searched for my aftercare Product, then dragged it from the Unassigned to the Assigned column.
Your customers will proceed through the Schedule Builder to the Addons screen. Each Addon Product available will be listed. If it is required, like with my uniform, it will have a label and your customers will need to add it to the cart to be able to checkout. If it is not, like with my aftercare, then they have the option to add it to the cart.
Here, I am choosing both Addons, and then I am going to click the Save & Continue button to proceed.
Your customer can then decide to add additional Timeblocks, and if there are Addons available for that Timeblock as well, then they will see the Addons screen again. As the participant already has their uniform, they are not presented it again, however they can still choose the other Addon.
After your customer clicks the Checkout button, they will proceed to Checkout. The Cart will show the main Products, and they can be expanded to show the Addon products. There is red cancel icon to remove the Dependent Addon. As you can see, the Required Addon cannot be removed.
This is also on the Review Your Order section of the Process Payment tab of Checkout.
After they click the Process Order button, they will have purchased their Timeblocks and their Addons.
Please take a look at our general article about Addons, and our article about requiring something to be purchased once, which is useful for the camper uniform scenario.