The nornal procedure is that if a player already has a USCF rating, even a provisional rating or one from many years ago, that is the pre-event rating that is used. That’s what 'Once rated, always rated" means, at least in theory. (In practice, if the rating of a returning member is from before around 1987 and we don’t know to reinstate it, the player might be treated as if he was unrated.)
If an otherwise unrated player has a FIDE rating that the USCF knows about, we will automatically use the FIDE-to-USCF formula to initialize that player’s USCF rating. However, the TD has to make sure that we have the player’s FIDE ID in our records. (The FIDE ID and country of registry shows on MSA.)
If an otherwise unrated player has a CFC rating, the USCF office needs to be notified of that so that the player’s initial rating can be set manually based on the CFC-to-USCF formula.
There are no conversion formulas in the ratings formula for other national ratings systems. However, the USCF ratings department has the discretionary authority to use that information to initialize the player’s USCF rating, but, as with the CFC rating, the TD will have to notify the USCF office in order for this to happen.
In special cases, when notified that a USCF rated player’s current FIDE rating is much greater than his latest USCF rating, the USCF ratings department can change that person’s USCF rating and rerate any events.
However, that only happens after the event is initially rated and then only if the USCF is notified of it. In every case I know of when the office has done this, the player has been from some country other than the USA.
One example of this was a player who had an 1800 rating from around 2000 but is currently a foreign GM. In theory this kind of post hoc ratings change could happen for a CFC rated player, too, but I do not know of any instances of that.
The constant thread in all of the above situations is that the USCF office must KNOW about the situation in order to do anything about it.