Is it possible to create Constants at runtime during a process execution? T

With the example you've provided, I would recommend using the database as the best approach. You can populate drop downs directly from a database using query rules. You should be able to achieve your requirement using a single table that contains a key to identify the 'constant' you're trying to define, a label column that holds the label values and a value column that holds the corresponding values.
Define a CDT to map to this structure and use query rules to pull the data you want.
Now, you can build a process to access/store or update these constants rather than maintaining an excel document which is prone for error when multiple people end up maintaining multiple versions. With the DB approach, you can maintain a single source of truth.

With the example you've provided, I would recommend using the database as the best approach. You can populate drop downs directly from a database using query rules. You should be able to achieve your requirement using a single table that contains a key to identify the 'constant' you're trying to define, a label column that holds the label values and a value column that holds the corresponding values.
Define a CDT to map to this structure and use query rules to pull the data you want.
Now, you can build a process to access/store or update these constants rather than maintaining an excel document which is prone for error when multiple people end up maintaining multiple versions. With the DB approach, you can maintain a single source of truth.