Are you asking how this can be done in Doit? Jus drag it for the projects list in the main pane to the Someday heading in the left panel. It will then show up in the projects list as an inactive project.

Are you asking whether it is a good practice to declare some projects as Someday/Maybe? Sure, if either you are not certain you will ever do it (it is just an idea), or if it is impossible to start this project until certain other projects have first been completed, and you expect this to take quite some time.

So "Inactive projects" list is some kind of an extension to "Someday" list? I thought Allen was meaning moving them permanently from "Projects" to "Someday" (for clarity). It would be nice to have all "Someday" in one place, having project's tasks converted to subtasks.

Beware that Allen's GTD is paper based and LIST based. You can have as many lists as you want for projects or for Someday/Maybe and Areas of Responsibility etc etc. In computer apps, for natural reasons, it tends to be more firmly categorized. It do not have a problem with that as such.

I do lack an "inactive" (subsequent) state for tasks within projects, though, as we already have for projects within goals. This would represent tasks that i will definitely do, but I cannot say when because it depends on when other tasks have been completed. This is something very different from "Maybe, and therefore Someday" which are tasks or projects that I have not even decided that I will do, and this is what I use Someday for. I do not want to mix up things that I have decided to do with things that I have not decided yet whether I will do.

I agree that "convert project to task with subtasks" could sometimes be handy.