Regarding the pyrokineticist’s fire lash from Expanded Psionics Handbook,
Can you use weapon afire on a fire lash and does the 2d6/4d6 stack with or replace the normal 1d8 damage?
The 2d6 or 4d6 fire damage from weapon afire is definitely a bonus, so it would be added to whatever the weapon’s damage is. See
She deals an extra 2d6 points of fire damage.
The real question is whether or not the fire lash is a “weapon.” It’s certainly like a weapon, being held in hand, making attack rolls in order to deal damage, being eligible for Weapon Focus et al., and counting as a whip for feats. But weapon afire is not a feat.
Ultimately, you will have to ask your DM if the fire lash counts as a weapon for weapon afire. I would say it absolutely should and that it certainly seems intended to be, but the wording doesn’t quite come out and say so, so it’s worth asking your DM about.
(Please also note that pyrokineticist is an exceedingly weak class; you’re welcome to play one anyway but you should do so aware of what you’re getting into.)