Hello Robert-
Thanks for reaching out! You can definitely use your 3rd-Party theme with the BoldGrid Post and Page Builder for WordPress.
The theme design itself will be handled by Lambda, but when you edit your posts or pages simply change your preferred editor to the Post and Page Builder.
From their page on Theme Forest, it looks like the theme already comes bundled with the WP Bakery Visual Composer, so if the theme and builder are tightly integrated you may have better luck using those together.
To get started, navigate to Appearance > Themes and click on Add New, then click Upload to install your purchased theme.