You can open and save these files to the host via WOPI+.
The Add-a-Place file browsing experience and the Save-a-Copy experience are intentionally filtered to only these file extensions.
Local-only file types
These are non-collaboration file types that are editable by the Microsoft Office Desktop apps, but are not supported for coauthoring.
As noted above, these file types are filtered out from the in-app file browsing experience via Add-a-Place.
In the case of an open from the local disk (including from a sync-backed folder) files with these extensions will always open in Local Only mode. Similarly, if the user wants to save to a file of this type, they can use the Browse functionality to open the native Common File Dialog.
This will provide a file type drop-down (as shown in Excel) that's typical for the local file experience:
Strict Open XML Document format
Coauthoring is not supported for files originally saved in the Strict Open XML Document format. Attempting to open a file stored in this format will result in the error, "Someone has this workbook locked".
Unfortunately, it's not always easy to determine if a file was saved in the Strict Open XML Document format as these files have the same extensions as the default file types. This means that Word, Excel, and PowerPoint files saved in the Strict Open XML Document format will still have the .docx, .xlsx, and .pptx extensions.
If your users are seeing error this when attempting to coauthor, use the following steps to try and resolve the issue:
Have all but one user close the file
The remaining user should save the file in any format other than Strict XML
All users can now open the document and coauthor
More information about coauthoring and Office file types
Demonstrate that you have the skills needed to get the most out of Word (Microsoft 365 Apps) by earning a Microsoft Office Specialist (MOS) certification.