How will you use this information that is now all in one field?
If you use a relationship to match to the desired subset of records, (Can you list them in a portal?), The list function can be set up like this:
List ( RelatedTable::Field )
and you'll get a return separated list of all the values in field over all the related records. Substitute can be used with list to replace the returns with other characters if that is necessary.
The ExecuteSQL function can also be used to generate such a list if you are using FileMaker 12 and this opens the door to listing the results of a full up query of your data in SQL.