Hello Maxim Ivanov,
I would recommend looking at the following thread (link below), there is no way to change/extend bootloader in ROM, but you can use a second bootloader, which perhaps behaves almost identical as ROM bootloader, if this is really needed.
https://community.nxp.com/message/757105
You may find an example of a secondary bootloader on Application Note 10711 (link below), perhaps you can use this as a starting point.
http://www.nxp.com/documents/software/AN10711.zip
I hope this information helps!
Regards,
