I think the English Equivalents to your two script steps are Set Field and Insert calculated Result.
Set Field need not erase all existing data in a field. To give one example:
Set field [Table::targetField ; Table::TargetField & ", " & //put expression for new data here ]
will then append data to the target field instead of overwriting it.
Note: Using Windows XP, FileMaker 12.04, I can reproduce this behavior. Insert Calculated Result ignores the focus on the current portal row.
Thanks PhilModJunk for the workaround.