Why do you want to mess up a ”nice vintage bike” by retrofitting a frame member?
Unless that ”accomplished welder” is willing to work for free, this idea is likely to be financially stupid.
For casual riding, there’s very little practical benefit from adding a top tube to a step-thru frame.
For enthusiastic riding, you’d be better off with a frame built for that purpose from the start. So I’m sorta struggling to see why you’d want to do this. Anyhow, most Dutch bikes are quite heavy, due to having quite thick tubes. Welding on them is relatively easy compared to higher quality frames. I wouldn’t want to try it with a stick welder, but should be entirely doable with a wire-feed MAG or TIG. Brazing is certainly an option too, although it’ll mean a bigger area to repaint.There’s always a risk for local warping right at the weld. The ability to ream seat- and headtube after welding is advisable.
If you only want to make the bike LOOK like a man’s bike, buy some round stock at the lumber yard, or a length of PVC tubing at the plumbing section. Trim to fit, and secure to the bike using zip ties or pretty much anything.