Hello
The unallocated space is located in a totally different disk from the target disk, right?
If that is the issue, I am afraid it is not available to do that.
The built-in disk management only support to extend the disk space right after the hard drive letter. If the unallocated disk space is in the same disk but it is not after the target drive. We may consider the third party disk tool, you can goolge the related tool such as "disk genius" tool.
Best Regards,
Wesley Li