I am currently serving as a guest editor for a special issue at MDPI. Some of my students got really excited: Can we submit something there? I am not really sure why they think this is a good idea and in fact, some people do this. However, my simple answer is: No. It is not a good idea. First of all, if the journal and /or publisher are sensible, they will require a completely independent review process for your papers. They will need to find another editor for this job. At the same time, if it gets published, that process is not visible any more and people might think: Oh, how unprofessional is that, publishing in your own SI!
Remember, your scientific reputation is all you have. Do not put it on risk just to get another paper on your list.