I have a UR10e, in which i would like to add an attachment to one of the arm, It weighs around 3 to 4 kgs, which moves along with the robotic shoulder. I need help about how to approach to create a solution with respect to estimating the center of gravity, axes and center of mass while assembling this attachment to the shoulder. How does weight play a role in making the attachment be mounted on the shoulder length. Does it really need to be measured for acceleration adjustments of the robot and weight distribution of the robot for better precision or posing? I dont have idea. Please help me out here. 
What is the final application of the robot? How much speed and precision do you need ?
I have attached similar weights to different areas of the robot as well as cable/pneumatic/hydraulic wiring harnesses along the entire length of the robotic arm that had a similar weight overall.
In those applications (pick and place, visual inspection, 3D-Printing) the weight did not change the dynamics of the robot in any noticeable way, but it is generally not recommended.
yes. I would like to use it for welding purposes. so the wire feeder system or the gas pipe are considerably heavy
For cables running along the arm, it’s not a problem, because the weight is more or less distributed over the entire robot and since the weight / CoM of each robot segment is higher relative to the cable and the distribution of the cable weight gives a roughly equal CoM. For a bulk weight such as a welding wire feeder, this no longer holds. UR does not recommend placing this on the arm (upper or lower), as it would negative impact the system (i know because I asked). However, you can edit the CoM calibration files on the robot to compensate. But I would advise to look for a different placement of the wire feeder. Some systems out there use a gallow and balancer.
How do i edit the CoM calibration files? can you please elaborate? I know the mass and its CoM, so based on the resultalnt CoM , how can i change it? Thanks in advance