When uploading comics or other posts that have existing pools in the site. I think it will be very convenient to directly add it in the pool when uploading.

Thats already possible. Just add pool:123 during upload and it'll be added. You can get the number by going to the pools page, https://e621.net/pools/22979 for example.

You can also use the pool name (with spaces swapped for underscores). This even works with autocomplete - just type "pool:" followed by the first three characters of your pool's name, and you'll see a list of options.

I veer back and forth between thinking this should be made less of an Easter egg feature due to its obvious use for the many people who post comics here, but at the same time being keenly aware that there's enough of a problem with vandalism of old pools already.

I had no idea that it could already be done (although it doesn't surprise me because I knew you could do it with the set: tags)

I still think it would be nice to have one like on the post page, complete with the search.