I have created a similar site. At that time, main-site was MODX and sub-site was WP. Basic processing will be as follows.
- Make new theme layer plugin
- Require wp-load.php at the beginning of the plugin
wp-load.php is the same as qa-base.php in Q2A. You can call WP functions from external system by including this file.
- Override the appropriate layer function (style, header, footer, etc)
- Call WP function (wp_header, wp_footer) from overridden function
It will not be enough to just include WP header and footer HTML in Q2A. You will need to include also WP stylesheets. WP stylesheets will conflict with Q2A stylesheets. You can avoid this conflict by devising Q2A style. However, knowledge of style sheet is necessary for this work.