Hi - a few points here:
As you say, overseas runners with no form in this country are relatively rare. However, when they do occur there are a number of common form elements that can often be used - in particular, trainer form in this country, sire and dam_sire form, jockey form and so on. Usually, there is also a form string to represent the places achieved in recent races, albeit not necessarily in this country, as well as the number of days since a previous run. Again, all of these elements are generally significant, certainly in model building, although the situation is not optimal for these particular runners.
Taking the example of the US runner Kaufymaker on the first day of Royal Ascot 2021, we have the following:
Trainer: Wesley Ward Owner: Mr Gregory Kaufman Ridden by: John Velazquez Cloth number: 17 Stall number: 3
Age: 2 (b. 2019) Colour: ch Gender: f Weight: 8-12 Bred: USA
Dam: Heaven's Touch (b. 2010) Sire: Jimmy Creed (b. 2009) Dam Sire: Montbrook (b. 1990)
Form: 1 (FlatSix) Forecast price: 5/2 Days since ran: 61
Thus the only form missing are the details of the race that Kaufymaker won when running previously at Keenland. - The other form elements are all useable.