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When you're looking at some action that you've realized needs an additional action added as a [pre|co|post]requisite, bring up the editor/inspector, switch to the Info panel, and tap the curved arrow next to the project name and select Go to Project from the resulting popup. That will take you to the containing project. Tap the + at this point, choose New Action in "<project name>" and enter your action. Tap the Edit button and drag your new action to the desired spot. Tap Done.

Another possible approach, if you're good about keeping on top of overdue items: use quick entry to toss the new action in the inbox, but give it a due date/time of now, or some time before you're going to start working on the project to which it belongs. This will give you minimum disruption of your current activity, but still puts up a flag of sorts that the new action needs to be properly incorporated into the project. The assumption here is that when you finish up with the current activity, you'll have a look at the Forecast view (or at least the sidebar) and will notice the presence of an overdue action.