Both of these are possible. To install courier-imap, you'll need to remove the e-smith-wu-imap package (I think that's the name of it), and also remove /etc/pam.d/imap. It should then install just fine.
To install IMP 2.3, you'll need a few more things. First, you'll need the latest PHP RPMs--I grabbed them from rpms.arvin.dk. Second, you'll need CVS (to get the IMP files with)--you can get this off a RH7 CD, or from any RedHat mirror. Third, you'll need the latest PEAR files from PHP--I don't know of a good source for these by themselves; the only way I know of to get them is to download the latest PHP snapshot from snaps.php.net (or the CVS code), and build it from there.