Você pode usar a opção /append
do ImageX para isso.
De link
Append is a standalone function of ImageX that will add the contents of a disk volume to an existing WIM file. Let's say you've created an image of a C: drive containing an operating system, preinstalled applications, and desired configuration. You add a D: drive containing a variety of data files and perhaps some additional application packages. Using ImageX with the /append option lets you add the contents of the D: drive to the WIM file of the C: drive--combining two volumes into one ImageX file. A sample command-line to append images using ImageX would be:
imagex /append d: c:\data_drive.wim
From the appended WIM file you can then apply either volume 1 (the first drive in the appended image) or volume 2 (the second drive in the image) in two simple steps without switching WIM files.