Please help us understand the use case you have in mind. The whole purpose of the InBox is to notify you of newly available podcast episodes from your subscriptions, so I’m trying to understand the rationale for wanting to manually add episodes there.
Well, it’s very possibly just me but I use my inbox as a place to keep all the episodes I’ll want to listen to at some point and the queue for episodes I want to hear in the pretty near future.
You may want to look at using Tags to organise your subscriptions and episodes into more manageable groupings if you find the Episodes screen is too cluttered even if filtered to show just unplayed episodes. For example organise by genre such as News, Sport, History, Science, Culture, Drama etc etc.