Whether it can work depends on the format that the pop server wants to see for the mailboxes. If it demands that the mail be kept in its own format you will need some automatic translation program or you are out of luck.
If the pop server is looking for the standard unix setup of a mail directory (/var/spool/mail or /usr/spool/mail) and each mailbox is a text file named with the user name, you could install Samba on the unix box (making it look like an NT server to the NT box), share out themail directory to the NT box, and map the share to a drive letter on the NT box. Point your pop server at the drive and it should work.
WARNING -- this solution will require that some user on the NT (the one running the pop server) gets root access on the Sun. This opens a possible entry into your Sun box.
Good luck.