Consider this F# code to sum the numbers below i that are multiples of 3 and 5:
let isMultipleOfThreeOrFive n =
(n % 3 = 0) || (n % 5 = 0)
You did the best thing.
(There is not an easier way to "get around" it than by adding the six characters you added. Even without implicit conversions, the F# is shorter than the C#, and the delta change-from-int-to-BigInteger is also smaller in F# than C#. So don't be sad about the loss of implicit conversion - be happy about all the other succinctness wins. :) )