Can the system of votes (of Stack Exchange) be considered as a blockchain? When discussing with a friend about blockchain, he told me that he read an article about using blockchain in management: people can vote on somebody else's work, so that he can gain reputations points, etc.

This made me think immediately of stack exchange, with its voting system. Then I found this: https://followmyvote.com/online-voting-technology/blockchain-technology/

It is possible to hack stack exchange et add manually reputation points? Is there anything to compare with blockchain technology?
First off, no, I would not consider StackExchange to be a blockchain.  StackExchange is a centralized web site, just like most (arguably all) websites.

Many of the proposals people make for using blockchains nowadays don't really require a blockchain to work.  So while, yes, you could use a blockchain to create a StackExchange-like voting system...you probably wouldn't want to.

When deciding if a blockchain is the correct solution, ask yourself "can this project work if I have to trust a centralized authority?".  If the answer is "yes", then you probably don't want to use a blockchain.  A blockchain is the solution to the problem of needing a data store that you can trust, yet not being able to trust any single individual or organization to run that data store.

In the case of StackExchange, the maintainers of the site have a vested interest in making it useful, informative, and collaborative.  If they don't, nobody will use it, and they won't make any money from advertisements.  Therefore we can trust them to do a good job maintaining the site.  This is also probably true for a company maintaining a reputation system for their employees.

In the case of Bitcoin, we can't have a central authority that we all trust.  In the past, centralized alternative currencies were shut down by governments, so even if we trust an organization, it has a weakness.  Furthermore, how much more can we trust a centralized upstart currency than we can government fiat currencies?  Bitcoin needed a blockchain to work, because centralized trust does not work for the project.

Here's a couple of other relevant questions you may find interesting:

How to implement a Bitcoin voting system:

Blockchain-based polling/voting systems

...and why you shouldn't:

Blockchain Tools for Voting Among Small Groups?