Hello, Ilya.
I think you can implement such behaviour with setting of the krt_prefsrc attribute value inside of the export filter of a kernel protocol section.

Simple example:

protocol kernel k_main {
  scan time 10;
  import none;
  export filter {
    krt_prefsrc = 10.0.100.1;
    accept;
  }
}

Also, you can set the krt_prefsrc inside of the 'if' statement of comparing of the next hop attribute value.