Central Asia isn't going to be able to contribute all that much. For example, Kazakhstan's and Uzbekistan's annual military spending in the 1990s never got above $250 million apiece, and they are the two biggest military powers in Central Asia (in terms of economic assistance Kazakhstan's total foreign aid to other countries between 1991-2017 amounted to $450 million). Again, the Soviets were sending a little over $3 billion a year in military assistance to Afghanistan, so there's zero ability for Central Asia to make up the difference once Russia cuts funding.
In addition to what
@Death's Companion says about internal opposition to what Najibullah was doing making peace difficult I would add that on the mujahedeen side there was tremendous pressure from Pakistan to keep the war going till final victory. Pakistan had control of the money spigot, and repeatedly threatened to turn it off if groups started seriously negotiating with the government. This pressure is unlikely to let up, as the Inter-Service Intelligence Agency (which actually directed Pakistan's policy in Afghanistan) was determined to see Najibullah overthrown and replaced by an Islamist regime. Basically as long as Pakistan is in the picture the mujahedeen have a strong incentive not to negotiate, and even if moderates were to break with Pakistan the Islamists would still be very strong thanks to Pakistani aid.