Looking at the map, you could probably do it from just after Rainford.
If you were going to do that (which would be 100% crayonista), then you'd run a service from St Helens Junction <-> Central <-> Rainford to the new Skelmersdale branch. Of course this would take up a path on the Skem spur to be used by about 3 people a week (if that), and when it reached St H Junction it would have nowhere to go. Perhaps extend it to Warrington Bank Quay and that way you'd mess up the Skelemersdale spur, the Lime Street to Wigan services, the Chat Moss line and the WCML. Or put it to go to Widnes and then to Warrington Bank Quay Low Level to mess up the NPR route just for the lols.
But obviously nobody is going to do anything like that, ever.
Back in the real world, on the subject of commuting flows, it's worth pointing out that any potential demand is kaiboshed by the Kirkby split. Removing that split could free up some demand. Upholland station is badly located, but that's because the access is built onto a narrow country lane. Geographically it's located very close to (i.e. adjacent to) an empty industrial estate, and it would be easy to open an access route to that estate which has space for car parking and much more direct road links to Skelmersdale. Orrell station has no scope for car parking, but has a fair bit of traffic as has been mentioned for Winstanley College (which is a prestigious further education college, in the top tier of state sixth forms, and so attracts students from a wide area).
Pemberton station is going to have a link road built past the entrance and a new large housing estate built adjacent to it. There is plenty of space nearby for car parking that could be made available when the link road is built.
I'd also put in a new halt near the Saddle junction (behind B & Q), which whilst only a few hundred metres along the line from Wallgate, makes access to retail parks, industrial areas and the Wigan football / rugby stadium possible.
My tuppence worth is if the above took place and Merseyrail went to Wallgate, there could be a significant amount of traffic going to / from Liverpool. There just isn't at the moment because the service is so slow and irregular it isn't worth it. I think you'd get much more bang for your buck doing that than the Skem spur to be honest.