Zend certified PHP/Magento developer

how to update Busybox commands by modifying /lib and /bin directly?

I am developing on a router from Advantech that has manufacturer’s firmware built with BusyBox and LTIB. It’s built with bare minimum commands, and I want to extend the commands to meet some needs. Luckily Advantech provides firmware source which contain the BusyBox config file. I used that and compile the BusyBox with additional commands I need. However, the LTIB built firmware a user can compile doesn’t include their proprietary software. So I am unable to use the firmware .bin file compiled by LTIB directly because it’ll miss some other core functionalities, which I haven’t yet confirmed what missing functionalities. So I tried replacing /bin/busybox with the newly compiled one, but it shows segfault when I run it. So I guessed I had to modify /lib/ too, but while trying to modify /lib, I broke the system. Is there any safe way to modify /lib, /bin directly to update BusyBox commands?

A side note, I think the Linux on this device is just the Linux kernel, below is what is shows:

# cat /proc/version
Linux version 3.12.10+ (root@localhost) (gcc version 4.9.4 (GCC) ) #1 custom