Question about reinforcing a metal stair rail

We have a metal stair rail that need reinforcing in the middle. The rail has a newel at each end and the handrail is attached to a post in the middle. The string capping at the bottom of the balusters is suspended above the steps because there is no outside stringer.
We want to weld an iron bar under the string capping around the middle, and attach it to the step below with lag bolts. I only have experience with soldering, is this a possible diy project? Or if not, who should I hire to do it and how much should it cost.?

Thanks