Yes, you just have to replace the "webmail" dir with one of your own. Rename the current webmail to something like "webmail-horde" then create another named "webmail", then you can install uebiemaui or squirrle mail or something that can access via POP3. You may also need to modify the httpd.conf, in the past they have had it set to automatically go to webmail.domain.tld/horde/imp.
I have a custom build of uebimaui running on all of our PSA servers, windows and linux.
*****Just make sure that if you install updates to rename your webmail folder to something else and rename webmail-horde back to webmail till your done. Horde is part of the base PSA package so it will get reinstalled automatically, even overtop of your custom webmail folder.