I have a similar situation to what you are dealing with, a level traditional garden set up for rows. I have been double digging a section of the garden each year for some years now and sifting out large rocks and adding organic matter to improve the sub soil and moisture holding capacity of the sub soil. I knew nothing about Hugelculture but I was putting a large layer of leaves on the bottom of each section for the same reason, to hold mosture. I came up with that theroy on my own it turns out. I have simce made a hugelculture bed in one section and it worked well.
I dug down as far as I could through the cap stone rock on my hill
I try for 36" but in this section 28" was as deep as I could dig
I put down wood chips, then logs, then filled in around them with wood chips
Then the sub soil was sifted and put back in with layers of leaves and soil
When it was all back in the hole I put the top soil back on top and planted tomatos in that section.
I am also using the larger rocks from the garden to build a small wall and the rest of the smaller rocks as fill to extend a level area near my fire ring.
The progam has worked very well for me. I do not water the garden at all after planting. The tomatos grew great, and the green beans that were in the improved section were much bigger and healthier than the rest of that row in an unimproved section.