If you're approaching Mporokoso from Mpulungu or Mbala, then about halfway to Mbala from Kasama you'll pass the Nondo Mission. Continue for a further 18km towards Kasama and you'll find a turn-off right, to the west. This road is about 60km long and will take you through to the Kasama–Mporokoso road, joining it about 8km northeast of Kapatu Mission (70km from Mporokoso). This is a very rough road; 4WD is essential, and it's difficult in the wet season.
A further 15km along this Kasama–Mporokoso road (about 23km from Kaputa Mission and 55km from Mporokoso) is a left turn, heading southwest. This short cut joins the Kasama–Luwingu road. It's not in superb condition, but is just passable in a normal car during the dry season, though you would certainly need a 4WD when it's wet.