Copy sample.ximapd to ~/.ximapd.
$ cp sample.ximapd ~/.ximapd
Edit ~/.ximapd.
$ vi ~/.ximapd
Do not forget to change user and password!
Import old mails.
$ ximapd --import /path/to/mail
$ ximapd --import /path/to/dir
$ ximapd --import-mbox /path/to/mbox
Edit ~/.forward to receive new mails.
$ vi ~/.forward
$ cat ~/.forward
/home/username/mbox,"|/path/to/ximapd --import"
ximapd may lost mails, so you should copy mails to other places.