You can show us some images of the existing conditions which you describe, to aid in the advice forthcoming.

Keeping the boxwood hedge at that low height can often encourage additional growth lower on the plant. That the hedge has been shaded and leafless for some time tells me that you may not have live branches in that area you wish to fill in.

Pictures save us and you thousands of words...

Now that you mention it, everything behind the outer hedge looks pretty dead. Probably not much hope of that filling in. Seems my spring trimming will have to be rethought but I will await your input. Thanks!