Hi Eugenio,
If I understand correctly your problem: on the Business / Domain Analyst side you have a domain-friendly set of enumeration, with the whole DMN model designed around it, as one would expect. On the other side, external clients might be using technical codes instead, in order to communicate the in/out values.
You need a way to bridge and make the translation between the domain model, expressed by the BA, and the external app or 3rd party app.
You may follow 2 general guidelines
Option1
You design this translation in the DMN itself, for example, you make first a subdecision that converts, from the technical code (US) received in the InputData, to the domain (United States).
This could be as simple as a common lookup of a Relation in DMN (something like lookup[code=myInput][1]).
This option is preferable when you want the Analyst to be in direct control of the translation.
Option2
You put a thin layer in front of the decision service implemented with DMN, where this translation happens.
It can be a simple Java code, orchestrated by a simple straight-through process with BPMN2 invoking first the Translation, and then the DMN model.
This option is preferable if the boundary of the Analyst is responsible for the DMN modeling, but coordinates with some more technical actor for the integration of the decision service to the calling application.
Hope this helps, let us know your feedback
MM