Set password for admin (FOnlineServer.cfg -> Access_admin)
Login and use ~getaccess admin [your password] command
Find prototype id (protoid) of your item; can be found in few places, Server/scripts/_itempid.fos, checking item in ObjectEditor, one of Server/proto/items/*.fopro files)
Use ~additemself [item protoid] [amount] command
Enjoy
If you're familiar with russian, there's Docs/FOnlineRu.chm where (probably!) you'll find more detailed instructions
If you're not familiar with russian, run Docs/Source/BuildEn*.cmd what should generate docs in english language.
Docs aren't updated frequently, and english one are always more outdated than russian, but such basics should be covered there i think.