Rebot into recovery.
Mount the system partition using the recovery GUI or through either of these commands
adb mount -t yaffs /dev/block/mtdblock5 /system #mounts only system
adb mount -a #mounts all partitions in /etc/fstab of the recovery
Push the apk to your system app folder
adb push C:\path\to\local\apk.apk /system/app/
adb push /path/to/local/apk.apk /system/app/