How does one read an external table to obtain the data requi

I would like to read an external table from within the corticon rulesheet inorder to extract data back and use this as the reference data to which the attribute is compared.

For example:

1

MyAttribute                                        {A,B,C}

myNewAttribute                                     'Z'

So I want to use a method of obtaining {A,B.C} dynamically from an external table so it is able to be easily updated.

Please let me know if there is a way to do this and if so how I would do this?

If not please let me know that too, thanks.