You can not just take any html third party template witch is not compatible with Blocks and just import it, there are few ways to do this:
1. You can add any html code compatible with the bootstrap or your own html code inside the block sections through a code editor, as well as add your own css styles and even a simple js code.
2. You can use the templates from the collection, change them and save your own version.
3. You can create templates from blocks, modify them and save as your own template than reuse them on any new project.