This "Grand Jirga"as it is called can actually amend the constitution so if a deal were struck to share power with the Taliban the constitution could be amended to make this legal. However the Taliban have always insisted on foreign troop withdrawal before they would negotiate anything of this nature. The US and NATO have tried to woo some of the Taliban away from the main group and reward them for supporting the Karzai government but this has so far had very limited success. Possibly the Taliban might participate if the negotiations involved a time table for foreign troop withdrawal but that is about the most that Karzai can expect I believe. But who knows for sure!
These excerpts are from antiwar.com.
Spokesman: Karzai Could Call ‘Grand Jirga' with Taliban
Massive Assembly Would Aim for Reconciliation
by Jason Ditz,
With his inauguration into his second term in office safely on the books, Afghan President Hamid Karzai appears to be looking forward to tackling his promise of attempting reconciliation with the Taliban, and spokesmen say he is mulling calling a "Loya Jirga."
The Loya Jirga, or grand assembly, is the largest theoretical tribal meeting Pashtun groups can hold, and the Taliban leadership would be invited to participate in this one.
Though unlikely, this could have enormous potential ramifications, as under the Afghan constitution a Loya Jirga is empowered to amend the constitution as well as make decisions in the national interest, suggesting that the Taliban could come out of such a deal with some formal power-sharing arrangement.