Not really feasable without moving the drain. Best done with a trench drain and zero entry to be ADA compliant. It's within the realm of any DIYer who's not afraid to tackle plumbing.

I'd say a prefab pan isn't feasable without moving the drain, as prefab pans are manufactured with a set slope to a set drain hole. You could get a custom pan fabricated. But it'd probably be less expensive to move the drain.

You could build your sloped bed out of deck mud and customize the differing sloped segments of the floor to fit your existing drain location, all while maintaining a code compliant slope. That could result in the elevation of the floor mud where it hits each wall being different on each of your four walls. That's not a deal breaker. Just something you deal with with these things.

You could leave the drain where it is and as hollysprings wrote, use a trench drain. With a large expanses of sloped floor leading to a trench drain, you can slope the floor yourself with deck much, or you can buy manufactured sloped pieces to create the slope on your flat subfloor. There are different versions of the "presloped panels" from different manufacturers.

A couple of things to keep in mind:

1) Your floor framing. If you have an open joist bay under the shower floor oriented in the correct direction so you can simply move your shower drain over 18" while staying within that joist bay, that's a pretty simple thing to do. If you'd have to drill through a floor joist to move ti that 18", that's a bit more effort.

2) Drain size. If your shower is old, it might have a 1-1/2" drain. Code updates now require a 2" drain.

3) In large tiled showers I recommend a topical membrane; Hydroban, Kerdi, RedGard, etc. With topical membranes I recommend using a flanged drain; Schluter/Kerdi, or Laticrete/Hydroban.

If you go with a mud slope, I do recommend you consider a Laticrete flanged drain and using Hydroban paint-on waterproofing membrane on the floor and walls of your shower. I think Hydroban is a bit more DIY-friendly than Kerdi.

If you consider a trench drain, also look at Laticrete's linear drains. They're a bit more price friendly than some of the others out there, and they're designed to work with Hydroban.

If you scroll about halfway down this thread, you can see a deck mud sloped floor with a non-centered flanged drain. This shower was about 5' by 8'. The Laticrete flanged drain is very similar to the Kerdi drain used in this shower. And instead of the Kerdi used in this shower, you'd use Hydroban. Different, but somewhat similar.

Here's a video showing Hydroban with a Laticrete linear drain. The drain doesn't have to go against the wall. It can be within the field of the floor.

So...you have options.

The shower floor, liner and plumbing are not something I would recommend for a DIY. ItâÂÂs not so much that it couldnâÂÂt be done but itâÂÂs more along the line of âÂÂitâÂÂs harder to do properly with no experienceâÂÂ. Even if you do not wish to move your drain itâÂÂs best to get a plumber in to inspect .My shower was a previous DIY and the slope was so bad itâÂÂs hard to describe. On top of that there is some expertise involved with the liner and liner corners. If I were you I would hire a plumber to examine the drain and put the liner in after a tiller does the first hit of mud. After the first hit of mud and the liner the shower floor gets the final mud from the tile guy. Next they do the tile on shower floor. You can do everything else but I wouldnâÂÂt mess with that part. ItâÂÂs very unlikely you will find a prefab shower pan to work with your drain as is.