Google is your friend. Googling system 2000 commands returns over 16 million hits, and on the very first page is a link to the Getting Started with S2K manual from SAS. And chapter 4 in that manual talks about the INSERT TREE command and its syntax.
If your site has a license for S2K, then you can get the manuals from SAS. Unless it has changed a lot since I last used it, S2K is a hierarchical data base not a relational data base. Hence the command syntax tends to be different since it was around before SQL.