This is sometimes called a multi-value key and is used as a kind of "short cut join table" for implementing a many to many relationship by using it as a match field in relationships. Used in that manner, a record in the related table will be linked if it's match field matches to any one of the listed ID numbers.
If that is how this field is being used, why do you want to combine the ID's in this fashion? It could produce fields with very long lists of ID numbers and will affect how any relationships based on this field function.
In most cases where I'd encounter such values, I'd give careful consideration to the possibility that the data needs to be moved into records of a related "join" table where you have one record for each value as it provides additional flexibility when working with this data.
That said, you can set up this self join relationship:
YourTable::FIeld_ID_Related = YourTable 2::Field_ID
and use it in this script:
Show All Records
Go to Record/Request/Page [First]
Go To Related Record [Show only related records; From table: YourTable 2; Using layout: "YourTable 2" (YourTable 2) ]
Replace Field Contents [No Dialog ; YourTable 2::Field_ID_Related ; List ( YourTable 2::Field_ID_Related ; YourTable::Field_ID_Related ) ]
Go to Layout [original layout]
Go to Record/Request/Page [next ; exit after last]
This code combines the data already present in the field with the new data from the parent record. It does not attempt to filter out duplicate values in this list of IDs.
Thanks for the technique. With a few adaptations (due to specifics of my database), this works flawlessly.