I just wanted to post what was my experience so far.
1. Creating a QEMU image of RouterOS turned out to be quite an easy task. I followed the description in mum.mikrotik.com/presentations/ID13/rofiq.pdf
2. Putting the RouterOS image in GNS3 is also a matter of setting a path to the QEMU image and setting some parameters. it's great that I could put as many interfaces I need.
3. Licensing of the image also works. I just tested it.
What I still haven't figured out yet is how to keep the license. Every time I create a new topology I have to enter the license...
Other than that - it all works great