You can do this by:
1. creating a grouping (not for the field you wish to display horizontally).
2. Create a formula something like this:
WhileReadingRecords;
StringVar sID;
If Len(sID)+Len(ToText({Your#},0,'')) < 250 Then -- Note: your string can NOT exceed 254 characters. If it does, you can create additional overflow variables (sID2, sID3, etc))
sID := sID+' '+ToText({Your#},0,"")
3. place your formula in the footer section and format the field to 'Can Grow'
4. run the report. If you wish, you can hide the group header as well.
I hope this helps. Good luck!