Define Drop-Down list of integers

There is an attribute type Integer that you can use for integers. There is no option to make that a drop down selection. You have to type the integer. 

I do not think Excel cares about the type, it cares a bout the representation. If you want a drop down, you have to use an enumeration. 

If you want to use an enumeration, carefully test on a test machine, what gets exported. I think it is the (string)value of the enumeration.
You can also define an enumeration in a way, where the literal ID is represented by a number, and the display value looks a bit different. In order to get the result like below, you have to define the enumeration literals and then go into the process configuration source to manually modify the enumeration literal ID values to what you desire. As an example look at the built in complexity