is there a simple way to add shortcuts that puts the current email into
a specified folder? What I have in mind is pretty much like tags, but
with folders instead (I use mutt interchangeably with Thunderbird, and
mutt doesn't support imap tags, so I want to use folders as tags).

For example, I'd have a folder "Important", or "Reply Later" on my Imap
server, and when I hit '1' on an email, it's moved into the Important
folder, '2' into the Reply Later folder etc. As a matter of fact, I'd
love to use 1, 2, 3, etc for my purpose, which are the shortcuts for
tags by default.

Instead of having the shortcuts mapped to "move to folder X" directly, I
could also leave in the standard tagging feature, if there was a way to
move tagged emails to a specific folder automatically. It doesn't seem
like standard filters can do that.