Adding external tables to your report

[manita]

I have a report on which I like to add an attribution (extra field) that can't be found in the database. The attribution that needs to be added, depends on the value of one or more fields.

Example:
Field1 = 1 > attrib = A
Field1 = 2 > attrib = B
Field1 = 4 > attrib = A
Field1 = 5 > attrib = C
Field1 = everything else (including null) > attrib depends on field2

Field2 = 1 > attrib = C
Field2 = 2 > attrib = A
Field2 = everything else > attrib = null

I like to use 2 external tables for that (or maybe I can build them in my report?).
I never worked with external tables or something like that, so I have no idea how to do that. Anyone here can tell me? Or maybe someone has a better idea how to do this? The table can change every now and then, so it needs to be flexible.