In a medium bowl, whisk together the vegetable oil, jam, vinegar and soy sauce. Add the chicken and toss to coat; refrigerate for at least 30 minutes or up to 4 hours.
Transfer the marinated chicken to a bowl. Pour the marinade into a small saucepan, boil for 2 minutes, then let cool.
Preheat a grill or grill pan to medium. Thread the chicken, plums and onion onto 8 skewers. Grill the kebabs, turning occasionally and basting frequently with the marinade during the first 6 minutes, until the chicken is cooked through, 12 to 15 minutes.
Meanwhile, in a medium saucepan, bring 2 cups water to a boil. Stir in the olive oil and then the couscous, cover, remove from the heat and let stand for 5 minutes. Fluff the couscous with a fork, add the almonds and toss. Serve the couscous alongside the kebabs.