Help me understand themes with sidebars and how to populate the sidebar from drupal
In Drupal, themes have regions. The sidebars are regions, just like the header, footer, conternt etc regions. Over simplifying a bit, content is places in blocks, then the blocks are placed in the regions, and changing the theme usually requires the blocks to be allocated again. Fields are part of the content, not the theme. There is no 'sidebar field'.

I would suggest that you set up a drupal test-bed, and play with it for a while before you take the leap of trying to migrate a site. Drupal is powerful, but there is no way to bypass the learning curve. You need to roll up your sleeves and work with it for a while.

Good luck.