Use the CONCAT action, select the input fields in their correct order, specify your separator, and select OBX.5 for the destination.
If you really want to use OBX.5 subfields, since there are not usually subfields in OBX.5, just do a normal COPY from each of your source to OBX.5, which is essentially subfield 0, and manually change the subfield index on the 2nd and subsequent COPY lines to 1, 2, etc.
