Guys, as far as I can tell, the ability to craft Stimpaks is not scripted yet, but you can edit Chems_Stimpak.xml in \Steam\steamapps\workshop\content\294100\725471844\Defs\ThingDef_Chems.

I did this: Open Chems_Stimpak.xml in a text editor like Sublime or Notepad++. AFTER YOU MAKE A BACKUP OF IT!!!!!!!!!

1.Make a space between <statbases> and <marketValue> and insert this: <WorkToMake>380</WorkToMake> (You can substitute 380 for something higher or lower to incresase or decrease the time it takes to craft)

2. Between </baseIngestTicks> and <ingestHoldUsesTable> insert this:
<ingestSound>Ingest_Inject</ingestSound> (This gives a sound effect when stimpack is used)

3.Between </ingestible> and <comps> insert this:
(This is what will let you craft the darn thing)

I've only done this for stimpacks and not super and ultra. It seems like stimpacks are pretty powerful and valuable. This will likely unbalance the game in your favor without further editing.