pitufan, you can already edit multiple meta keys for a single user, or custom meta for all users at once.
Click the User Meta Manager Help tab, then Plugin Settings, then for Meta Editor Shortcut, select Yes.
So, next time you edit a single user’s meta data, you will see ALL the meta keys at once, instead of selecting a single key to edit.
That method is for a single user.
You can update a custom meta value for all users at once by going to Edit Custom Meta, then select a meta key to edit, then check Yes to Update Value For All Current Users.
That method will update custom meta data for all users at once.
Just for the sake of mentioning, only custom meta data which is added via the main Add Custom Meta button can be edited via the Edit Custom Meta button, or deleted via the Delete Custom Meta button. If you use the links in the results table to Add Meta for a single user, you can only edit or delete that meta data via the Edit Meta, and the Delete Meta links, and not the main Custom Meta buttons. Actually, meta data which is added via the Add Meta links will not even be displayed in the Custom Meta screens.
Not all members have the same meta keys. That is the reason you cannot edit, for example, 3 or 4 members from the list at once. Those 3 or 4 members may not share the same meta keys.
I hope this info helps!