How to Assign Cost Codes to Extras?

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Can someone walk me through how to assign cost codes to extras?

We have a job that already has the (labour) cost codes set up, but those cost codes now need to be grouped together into "progress codes".

From what I understand, progress codes (which in our case a 'progress code' = phases of the job) are called "extras" in Sage.
So now I have my extras set up. Can I now assign cost codes to those extras?

So say we have cost code 100-1, 100-2, 100-3 and 100-4. Now we have entered Extra 1 & Extra 2 (AKA progress code/phase 1 & progress code/phase 2)

I want Extra 1 to have cost codes 100-1 & 100-2, and Extra 2 to have cost codes 100-3 and 100-4.