How to write in custome partition while build?

Hello everyone,
I have install openwrt in my board successfully. there are several partitions in os.I have created 2 partitions of 32 mb flash.first 16 mb is using in rootfs.But i dont know how to use second 16 mb.I am including some self created script in basefiles so i get in firmware.But how can i include files in other 16 mb partition???
following is my mtd table,

mtd0: 00040000 00010000 "u-boot"
mtd1: 00010000 00010000 "u-boot-env"
mtd2: 00e30000 00010000 "rootfs"
mtd3: 007f0000 00010000 "rootfs_data"
mtd4: 00170000 00010000 "kernel"
mtd5: 00010000 00010000 "art"
mtd6: 00040000 00010000 "reserve"
mtd7: 00fc0000 00010000 "user"
mtd8: 00fa0000 00010000 "firmware"

1st 16mb is rootfs
2nd 16mb is user.
i want to write data in "user" partition while building firmware.
I also need mount user partition everytime. is there any way where it mounts while loding kernel?