Does this approach work?
EDIT A shorter description:
1. To import the directory, drag-and-drop.
2. In the Album browser, sorted by Date Added, group the unrelated tracks: right-click, click, click, click; Edit; Group Albums
3. Give it a name, Frank’s Recommendations
I’m sitting on my Windows machine, where I have a subdirectory with unrelated tracks, as you describe. I’m looking at it in Windows Explorer, which is designed to browse directories. I drag the subdirectory onto the face of Roon, which downloads it to the server. If the tracks are identified as you suggest, if each track has metadata for album and artist, they will appear in Roon as a set of (partial) albums with the artist name.
(I also tried it with tracks that had no metadata at all, and they appear as tracks in an album which Roon made up, consisting of my computer name and date and time. And similarly if they had artist and track names but no album.)
In either case, in Roon’s album browser, Sorted by Date Added, the newly added tracks appear in one or more “pseudo-albums” in the upper left corner of the browser. I can select all of them, click Edit in the upper right corner, and select Merge Albums. Now they appear in one pseudo-album – “pseudo-album” is not an album at all, but it’s a grouping of tracks that you consider meaningful in some sense. You can give it a name, “House downloaded Dec 29 2017” or “Lucy’s recommendations” or anything that makes sense to you. Edit other metadata if you want.
Now, this grouping of tracks is available to you in all the normal Roon ways: you can play the whole grouping, individual tracks, add tracks or the group to playlists, etc.
Before you reject this as a complex workaround, think about what I’m doing here: the tracks originally didn’t live in a directory, they came from various sources. The subdirectory is a temporary holding place, just because Windows insists on keeping things in directories, but that’s a technical artifact. You use the Windows tools to manipulate that directory, but once the music comes to Roon, you can keep track of them in a more logical way. What the tracks had in common was something that made sense to you. Tell Roon about it, and move on. Forget the temporary storage location, it’s uninteresting.