My internship is going to sponsor my bachelor's thesis (which will require more hours in the office). Should I ask for more money? I have worked for the past year as an intern in a company that is partly based in my university. I have been receiving a scholarship for my work.

We agreed on how much time I was going to spend at the company and how much money I would receive. My scholarship amount was determined by the time I agreed to work monthly, at least that was how it was explained to me.

During the interview for this position, we agreed that I would do my bachelor project here, which basically means I will do the research they need and then write my bachelor thesis about it. This is a win-win for everyone, because instead of working ON TOP of school, I would work for the company AND work on my degree at the same time. The company benefits from this because it allows me to spend more hours weekly working.

My work on the bachelor thesis is about to start, which means I will be able to spend about twice the amount of time in the office than before. My boss and coworkers expect me to be more often in the office.

I would like to get double the scholarship, but I heard from someone who was in the same position as me in the past that his scholarship did not increase and he was told that he would have to do the work for his bachelor project anyway, so even though he spent more time working, he received the same scholarship.

This is basically free work for the company. I don't like working for free. Is this normal? I will try negotiating, but what should I do if the boss doesn't change his view? One thing I was considering was that if they don't increase my scholarship, I would spend the same time in the office as before and do the rest of the work at home/elsewhere, which is more convenient for me.

Everything is already agreed, I can't change my bachelor project (do a different one, for a different company) without delaying my degree by at least half a year, so basically the company now holds my academic progress hostage, which from what I have heard is the situation of some PhD students.
I don't think you have a lot of leverage here other than by disadvantaging yourself. 

First, scholarship money is not always abundant and people like to spread it around to help more people, rather than to concentrate it. 

And, you are getting something for your work, beyond the money. You are getting a degree. It isn't that they get your work for free. You made an agreement, I think, if I have a accurate reading here. Live up to it. 

I think your statement that the company is holding you hostage is misplaced. In fact, they are helping enabling your future. Even just providing a project that you can develop into a thesis should be worth something to you. 

I suspect that starting over with a different situation would be worse for you in every way: time, money, and more.