Unfortunately you cannot create an entirely new block for Minecraft (**yet).
This has to do with a limitation in the original Minecraft game that we have no control over. The mods we make can only change the way a "multiplayer server" handles the "game client".
The way the original game was made there is only a specific number of textures allotted for block textures; we can't add or remove them, but we can change them.
What we've done is provide an easy to use tool for changing a block texture, assigning that texture to a specific block, and automated the process of updating your private Minecraft server, hosted by us, with the new and updated texture pack; all with a single button click.
We feel that this greatly simplifies the process of creating a "custom texture/resource pack" for your private server.
We also have a tool for creating a custom character skin, and we have a few more exciting tools that you'll hear about from us sometime this year!
I hope this helps,