Not really sure. I managed to get in there after they bumped us on an (economy) flight to KGL and we had to wait around for 4 hours. If it was that big a deal, you could call the Nairobi call center and ask, but they would probably let you into the simba lounge

This is what their website says:

"Access to the lounge is exclusive to Business Class passengers traveling with Kenya Airways and its partner airlines : KLM, Rwanda Air, Air Zimbabwe, Air Malawi, Pakistani Airlines, LAM Mozambique Airlines, Precision Air and Sudan Airways."