When you publish to stemgeeks you are publishing to the hive blockchain.
Reposting old articles will be seen as trying to double dip the pool.

I don't use it, but I bet that posting there also shows up on your blog.
It came along while folks still had hopes for blogging on hive.
Like most h-e 2nd layer ui's, it is mostly abandonware at this point.
That is likely to reverse were the price of hive to go back up.

Stem is a hive-engine second layer token and does add additional value to your post rewards.
Afaik, you get stem only through using their interface to post.
It works like most of the other h-e tokens and can be bought/sold through tribaldex.com.
You can see what h-e tokens you have in the peakd wallet by clicking the h-e option from the list on the left.