How to make my sourdough sweet?
It depends on what one means by "sourdough."  Some people tend to restrict that word to a particular bread style, usually associated with "San Francisco sourdough" or some variant.  That bread tends to be a lean somewhat crusty bread, with sourness varying from noticeable to somewhat pungent.

But others use the word "sourdough" just to connote the use of "natural yeast" (as opposed to bakers yeast).  If your source was using "sourdough" in that sense, it's perfectly possible to make all sorts of sweet breads using natural yeast.  It was actually the standard way to make such breads before the widespread use of commercial yeast.

If you are going to do so, besides possibly adding sugar or some other enriching agents to match your memory (milk, butter/oil, eggs, etc.), I'd also recommend efforts to "tone down" the sourness in your starter.  Traditional French methods for natural yeast sweet breads tended to work on a multi-stage building system, where you'd start with a remaining hunk of dough from a previous batch (the "chef") and then gradually build it up in anywhere from 2 to 4 stages to the final dough size.  Each build would range from an intermediate to large dilution of the starter (to dilute any residual acidity in the starter), and you'd start the next build before the previous one had a chance to turn sour.  

By doing this, you maximize yeast growth (which occurs earlier in the sourdough cycle) while minimizing acid production (which makes the bread sour).  After a few such builds, you'll have knocked down the bacteria population -- which lead to sourness -- enough that the rising of the final bread will mostly be just yeast activity.  The final bread is often more flavorful and complex than produced with bakers yeast, but won't have distinct sourness.

Also, this multi-stage method is less necessary if you are baking bread on a regular basis, like a bakery might do daily.  As starters sit for longer periods without feeding (more than a few hours), they will accumulate more acidity.