Lets assume that you need to add a new value to combo-box control without any dependent logic. And you do not want to do this with Customization as you do not know that. And with Acumatica it is really easy to do.
Lets check an example where you need to add new source for leads. "Source" is just for informational purpose and does not have depended logic (in other words there is no different business logic depend on selected value). So that means that if we add a new value there we do not break anything.
If your combo-box has logic associated (fields enabled disabled / different calculations) than you may need to consider code customization instead of approach described here.
Here with Automation Steps we can add non-programmatic customization for some fields. Lets add source field for step Open. For our task step is not important as you can add value on any step, but for other tasks you may need to add fields/buttons on every step where you want to use it.
Hope it helps and have an easy configuration!