I'm not entirely certain what you're trying to accomplish, but the short answer is yes, you can dynamically allocate a pathname for external assets by storing the path in a field and then referencing that field in a calculation in the Storage tab of the field definition dialog.
There are several caveats associated with doing so.
1) The path must be a path that the server can find, not the user. Therefore, drive mappings (assuming you're on Windows) will likely not work. And permissions will be an issue.
2) If it's not a valid path, your external storage will fail. I'm not sure what would happen in such a circumstance. Regardless, allowing a user to just type a path is inherently dangerous. (Read: I wouldn't do it.)
3) To what purpose would you be doing this? Because allowing users direct access to externally stored assets will break the links to the database, thereby destroying the external storage capability altogether.
If you want the users to be able to, say, edit the assets, then you'd be much better off to flag the record, download it to the user's Documents directory, then upload it when edits are complete. The flag would indicate to other users that the asset is being edited and, therefore, is in "Read-Only" mode.