If you've already obtained a legitimate ISRC, then yes you can reuse it.
Remember, all ISRCs must be 12 digits in length, and will look similar to this: GBDIT1312345
If you do not already have an existing ISRC, we can generate them for you free of charge, so you’re covered either way.
You can add an existing ISRC when you're uploading your release via the Ditto Release Builder.
From the Release Builder, click the 'Create a New Release' button to begin the release building process.
Navigate to the 'Track List' tab. From Section 1, 'Tracks and metadata', you will be able to see a list of your uploaded tracks on the right hand side.
Scroll down until you find a box labelled 'ISRC code' (which will be located on the right hand side, next to the 'Copyright holder' box).
You'll have the option to either manually enter your ISRC or automatically generate a new one.
Once you've entered your code, remember to finish by clicking the 'Finished editing track button'.