It is possible to declare a tuple as the primary dimension of a table in a read block through the keyword as:

read "/suppliers.csv" as Suppliers [(Supplier, Location) as myPrimary] with
  Supplier : text
  Location : text
  LeadTimeInDays : number

A more complete example:

read "/skus.csv" as Skus with
  Id : text
  Supplier : text
  Location : text

read "/suppliers.csv" as Suppliers [(Supplier, Location) as sulo] with
  Supplier : text
  Location : text
  LeadTimeInDays : number

expect Skus.sulo = (Skus.Supplier, Skus.Location)

Skus.LeadTimeInDays = Suppliers.LeadTimeInDays