Can existing types be extended to work with Seq.sum, etc?

Been working with a lot of TimeSpans recently, and have a need to get sums & averages.
However, TimeSpan defines neither operator get_Zero nor DivideByInt, so Seq.sum and Seq.average can't be used directly with this type. The following fails to compile:

open System
type System.TimeSpan
    with
        static member Zero with get() = TimeSpan()
        static member (/) (n:DateTime, d:int) = DateTime( n.Ticks / (int64) d )

let ts = [ TimeSpan(10L); TimeSpan(99L) ]
let sum = ts |> Seq.sum
let avg = ts |> Seq.average

• Error: The type 'TimeSpan' does not support any operators named 'get_Zero'
• Error: The type 'TimeSpan' does not support any operators named 'DivideByInt'
• Warning: Extension members cannot provide operator overloads. Consider defining the operator as part of the type definition instead.

Is there some F# magic that can define these operators on an existing type?

I know the following will work (and should be more efficient to boot), but I'm still curious about the above so I can add it to my toolbox for use with other types.

let sum = TimeSpan( ts |> Seq.sumBy (fun t -> t.Ticks) )
let avg = TimeSpan( let len = ts |> Seq.length in sum.Ticks / int64 len )