We have done exactly that, just copy files from log directory and it starts indexing them itself. Note that depending on current log rate arrival and size of old log files, it may take considerable time to process them. For us it took 3 days to process one week of old files so we gave up on them.
Here is a one liner that was written to monitor indexing state How to quickly check log indexing backlog
That's on MDS though so for SMS you need to take away CMA loop
cat $INDEXERDIR/data/FetchedFiles | while read line; do if [ `echo $line | awk '{print $7}'` -eq 0 ] && [ `echo $line | grep -c " fw."` -eq 0 ] && [ `echo $line | grep -c "serialization"` -eq 0 ]; then echo $line; fi done;