How to update CDTs dynamically

I think this is a requirement which should be rejected. Going this road will get you into endless problems, bugs and debugging sessions.

Can you give us a bit of a background? Maybe someone has a good idea to solve the overarching business requirement in a different way.

Appian will not make any enhancements to the CDTs now. With the new 22.3 release, it was very obvious that they are planning to deprecate the CDTs. It might be possible that just like query rule, in the new 22.4, they remove the functionality of creating a new CDT but the older ones remain there and function as it is 

The thing is: What is the purpose if a dynamic excel file?
If two systems are communicating, you usually agree on a certain set of values which will be delivered in certain way and the type of reaction you can expect. so its predictable behavior. 
Example:
A web-api will accept a certain method like "get" and will respond to the input fields x,y,z, with the values a,b,c in the format of JSON.
 its clearly defined the value type and the format for the input and outputs fields. 

So transferring this concept to your case,
you are are expecting a kind of dynamic number of acceptable input values in the format of en excel-file
and the receiving system reacting dynamically to it.
That is not how "inter system" communication is working :) 
Stefan is absolutely right. That is not a requirement you should follow up on. Besides if its perhaps working one day with the available appian tools.