Everlast had a pedal with that kind of limit pot for their early machines. There should be some schematics and values to get you started on this forum if you search back on pedals. For starters measure the existing pot in your pedal and use the same for your second pot. As it is being used as a voltage divider, you probably want linear, but that is personal preference. Audio might give you more granularity in the area you weld in. There are a lot of pedal mods listed here, everything from simple mechanical fixes, to arduino based controls to add features.
Here is a basic idea of what will get you going. Original on the left and the second pot added on the right.
Last edited by Rambozo; 09-11-2016 at 08:24 PM.
Long arc, short arc, heliarc and in-the-dark!