This is far more trouble than it is worth… unless you know JUST how to do it:
From within your new Kmail installation, click Settings>Configure Filters. At the bottom of the Filter Rules-Kmail window that pops up, click the button labeled “Import.”
Now comes the tricky part: finding your old filters. Mine were on an external usb hard drive mounted at /media/disk/HomeDirBackup9/.kde4/share/config/kmailrc
You may have to do a filesearch for your old kmailrc file, but generally, in recent KDE distros I have seen, it is in the /home/username/.kde4/share/config/ folder… so you’ll have to find wherever that folder landed in your backup.
Once you have located the file, click on it in the “Import Filters” browser window, and if all is well, it should pull all of your old filter rules into your new Kmail installation.