A responsive theme is difficult enough to create, but it’s even more difficult to try and take it apart. Conceptually this is possible, bit it’s not pretty. You would need to copy several responsive queries for the navigation menus from the parent theme style.css file and add them to a child theme or other custom css (and maybe combine them), change the min-width/max-width conversion points, then remove the same media queries from the parent them style.css. The media queries in the parent theme will be reinserted if you upgrade the theme so you’d have to remember to remove them after the upgrade. You’d also need to copy one part of a media query out of the parent theme responsive.css file and place that in the appropriate media query you copied from the parent style.css file. After all that, it will compress the menus at an earlier point but, if you expand them, they still expand in their native form; i.e. on two horizontal lines.
There are plugins to create mobile views, like WPtouch Mobile, but I don’t know how well they play with a responsive theme.
