One of the convenient things about the ArduinoJson library is that it is a "header-only" library. As such, including it with your sources in your "custom" code's package should be sufficient (along with #include "ArduinoJson.h")
https://openwrt.org/docs/guide-developer/packages is one reference on how to create your own package and build it within the OpenWrt build system. Packaging guidelines is also very helpful