sstableloader only stream existing sstables. If you need to load data existing in another form (json, csv, whatnot), you need to first write the sstable(s) to load. The recommended way to do this is either to use json2sstable or to modify it if your input is not json. Modifying json2sstable is however more involved than it needs to be, you'll need at least some basic understanding of a bunch of internal classes (DecoratedKey, ColumnFamily, SuperColumn, ...). Even for json input, you can use json2sstable only if your json actually conform to what is expected and even then, good luck to someone that want to add counters.
This ticket proposes to add a simple interface to write sstables.