You won’t be able to mutate, but if the overall struct remains relatively small, you can trivially create a new struct with the updated internal values. For example, with StaticArrays, even though they are immutable they still achieve the same semantics:
a = SVector{Float64, 4}(1, 2, 3, 4)
b = SVector{Float64, 4}(1, 2, 3, 4)
a += b # => a = a + b
In the last line, however, a isn’t mutated it is reassigned.