There are projects that permit statically linking a traditional kernel with a traditional application into a unikernel. NetBSD pioneered this with their rump kernel build framework, and I believe there's at least one Linux build framework that mimics this. The build frameworks cut out the syscall layer; an application calling read(2) is basically calling the kernel's read syscall implementation directly. Often you don't need to change any application source code. The build frameworks handle configuring and building the kernel image, and statically linking the kernel image with your application binary to produce the unikernel image.