That's not entirely true. We have the MAG. It may not be rock hard canon, but it does give us an idea.
(All of what follows is according to the MAG. Treat it accordingly)
MAG Feruchemical nicrosil lets a full Feruchemist store investiture (the small i is intentional) by converting charges of another stored attribute into what I call "blank" investiture. It's basically power without a shape... you can tap it on its own but it'll do nothing.
In that sense it's a bit like the investiture one draws on when one is burning metal or writing Aons - an undifferentiated energy that needs the metal or Aon as a key to tell you what form the power comes in - except this power has no form.
The way one can use it according to the MAG, is that one can tap it alongside tapping another property like Strength or Health or whatever. This other property "tells it what to do" and the investiture counts as more of the other thing you tapped. In that sense it is a "wildcard" metalmind that can be used for anything if used properly.
There are a couple of other ways to store it outlined. If one knew how, one could store the Mists, and one could also store the power at the well. However, this would most likely be the same kind of storage we've seen above - converting a source of power into a blank energy - and wouldn't let you store the well to use it's power later in the planet moving sense.
That said, I assume this works both ways... if one could store enough charges of investiture, one could theoretically use it to pad out the power of the Well in the same way one can pad out their Feruchemical charges... though given the amount of power at the well, surely this is but a drop of water in the ocean. A lot of effort for very little return.
In terms of a Twinborn with Nicrosil as their Feruchemical metal, I am convinced one could use the stored investiture to pad out their burning, or something like that, but I've had no confirmation on that.
For more musing on Feruchemical nicrosil and the MAG, please see my "Nicrosil Feruchemy Masterpost" in my Sig. Feruchemical nicrosil is one of my pet favourite abilities...