Currently you do not have the option to do it yourself. For now what Station does is it run an algorithm that detects the apps you use the most often and only keep 8 tabs open at any given time. We believe it's best if we can do it intelligently rather than manually. But we also see that some power users would like to be given control over that. Ultimately if we do a good job at doing it intelligently, there shouldn't be any need to do it manually. It's still a work in progress.