You should be able to select a feature pattern created in the weldment machining environment to pattern a component, just as you can with a feature pattern created in a part.
While we're on the subject of weldments, you should also be able to share a sketch in the weldment machining environment like you can in a part - vote here:
https://forums.autodesk.com/t5/inventor-ideas/need-ability-to-share-sketch-in-machining-environment/...