I think in this case (maybe someone else can tell something different) you'll need a computer with VSE executing a mirroring task to this share, this will also create the sitestat file.
Once the first mirroring task completed ePO should be able to pull from the share
There are a few considerations here:
1. ePO master & distributed repositories are signed because they can contain more than just dat content, they can also contain product installs.
2. Source sites are not signed because they only contain 'public' content and are therefore accessible to all. (i.e. McAfee Commonupdater on ftp or http).
3. The expectation for a source site is that it is unsigned, which is why you cannot point the ePO master or a Distributed repository to be a source.
4. All sites (source, master or distributed) have control files such as catalog.z, sitestat.xml and replica.log and a very specific structure. This means a straight copy may not work because the control files are not updated correctly.
As mentioned your best bet here is to use a VSE Mirror task to set up your UNC share. The mirror this makes can be considered the same as an ePO source site in that it is unsigned and only contains content and not product install.
From the VSE 8.8 Product Guide:
Mirror tasks and how they work
The mirror task replicates the update files from the first accessible repository defined in the repository list, to a mirror site on your network. The most common use of this task is to mirror the contents of the McAfee download site to a local server. After you replicate the McAfee site that contains the update files, computers on your network can download the files from the mirror site. This approach is practical because it allows you to update any computer on your network, whether or not it has Internet access; and efficient because your systems are communicating with a server that is probably closer than a McAfee Internet site, economizing access and download time.