It has an external PCI-E 16x male port that goes into a small dock/riser where you can plug a graphics card for instance.
https://droix.co.uk/blogs/minisforum-b550-review/
A second review with a different conclusion: https://www.youtube.com/watch?v=ihcGnBjUAm4
Not so reasonably priced, so it maybe does not fit your criteria.