Question: Is there a tool like the subject for Solidity. Let's say I have a contract with import and that import has three imports etc. I'd like to combine all of them into one file and play with it in Remix, for example.

I know how to make it manually.

Answer: 
I'd also suggest you to try this - <http://remix.readthedocs.io/en/latest/tutorial_remixd_filesystem.html>

This feature is currently an alpha version but it may help you.
please have a look on : [solidity-flattener](https://github.com/BlockCatIO/solidity-flattener)

This script automatically traverses the dependency graph and outputs all of your imports in the correct order, ready to be pasted into the contract verifier.