There is no built in function to move resources from one game to another, you have two options:
1. From the kit, upload the tileset to forge yourself
2. Manually, from your game: File > Create New > Tileset : Enter name
- Import Tiles from Sheet: Click 'choose image'
- Find the tileset in %Installation Folder%\StencylWorks\games\theKitWithTheTileset\resources
- Choose the appropiate tile width/height
- Click 'add'
* I find option 2 really easy and lag/connection problems free