If you have to do flat-file to flat-file then using a Script Task might be the only option and the article linked to above is how I would do it. Just be careful, if your source flat-file is very large, the sort required before you can start doing the concatenation is a size-of-data operation meaning the entire flat-file will need to be in memory before the sort can begin and that can ause performance issues if you do not have the amount of RAM to accommodate the file plus the size required to do the sort. If that is the case you may want to consider putting this data into a database first, then re-extracting it to your output file. It's more network traffic and logging in the database but it may be more efficient in the end.
There are no special teachers of virtue, because virtue is taught by the whole community. --Plato