Export needed 1st person welder animation and textures (KF_Weapons_Trip.Welder_Trip, KF_Weapons_Trip_T.equipment. Welder_D and KF_Weapons_Trip_T.hands.hands_1stP_military_diff) then open it in 3Ds Max. More details here[metalmedved.com].
Add new welder model to scene. Previously it needs to remove all bones from model.
Sync model poses
Using Move, Rotate and Scale tools combine models together so that new one looks naturally in hand.
Export new model
Delete old welder and set new welder model material ID to old one. Combine effect mesh with new model. You may test results by playing animations.
If you are making gun weapon then pay special attention to backsight combining and separate weapon parts binding also (magazine, barrel, cartridge)
Export obtained model to Killing Floor. Read details here[metalmedved.com].
Making 3rd person model
Repeat previous steps for 3rd person model (KF_Weapons3rd_Trip/ Welder_3rd). If you are making dropped weapon it needs to take care about model static mesh also.
Make mutator[metalmedved.com] that replaces regular welder. Create new mutator with 3 files:
class NewWelder extends Welder dependson(KFVoicePack);