I'm considering getting the White March DLCs to warm up for PoE2, but I'm wondering: how does the DLC connect to the main content? Does it get added after the main story, or is stuff kinda inserted into the main game?
I've already finished the main game and I would rather not start a new run just to see the WM stuff.

Also, I'd take this as a bit of a litmus test to see if I will grab the DLC for PoE 2 or not. I don't need nor want anything that requires to replay the game.

White March adds a whole new area with new companions and quests but it needs to be done before you head down the pit on the Burial Isle. PoE saves a game before you go down (called ENDGAME I think) so you can load that up and do the White March DLC before heading back to pit to finish Sun in Shadow. If you've completely uninstalled the game plus saved games then I think you're out of luck and you'd have to play through again.

The White March DLC comes in two parts. Part 2 becomes available after the completion of Part 1 main story content.

Part 1 does not affect the base game outside of the White March designated areas, except for a couple of quests and the option for several base game factions to get involved in one.

The White March Part 1 is added to the game after the prologue and the White March region of the map and a couple of areas and quests that take place in base game areas become available as well.

While you can pursue the White March regions and quests at any time and play it paralell to the base game, the challenge is geared towards mid level with the difficulty spiking drastically in a short time to high and end level base game difficulty through the end of Part 1. White March Part 2 difficulty is scaled for the additional character levels it adds. So a low level party is going to have a tough time in White March. Furthermore, the couple of additional quests WM adds are straight up impossible to complete before reaching max level. They are intended for end game.

EDIT: You may take White March companions into the base game.

Hmm...thanks guys.

So, it sounds a bit like a mix of both the layouts I mentioned. It's not strictly set after the finale of the main plot, but can (correctly if I'm wrong) be played from around just before the end.

Seeing that WM2 even expands on the level cap, how does play into the end of the main plot? Does it make things much easier? (Not the main plot was terribly hard anyway, I seem to recall)

All in all, I'm not sold on this. I want to start playing PoE2 rather soon, but I'd love to see the content of the DLC. I am, however plain unwilling to replay the main story to any (more than minimal) extend.
Given what I read here and on a couple of blogs and reviews regarding PoE / WM I think that combo is just not realistic, which makes me a tad sad.

There is an optional re-balance. If you finished the game before playing the DLC, once you arrived to the first area of the new DLC, the game will offer to re-balance t he enemies to your current level.

Similar, if you finished the Expansion before ending the game. The lasts areas, and the infinite paths, can be re-balanced.

You should be OK. You load the game slot before entering the final area. Go to to the keeper to start the quest, a fast visit to Defiance bay, and then you are free to go to the new DLC.

As has been suggested, just use the save from before the end of the main game and go to the White March content. I really enjoyed WM1 and 2 and highly recommend playing it.

Alright, that sounds a bit more to my liking then.
Really, what I'm trying to work out is: Do I sign up for the season pass for PoE2 right away or not. I will not replay the game for any DLC, so the DLC has to be structured in a way that I don't have to.

Those last two posts sound like I'd be fine.

Just to add to what the others have said, White March is one of the best expansions I played in the recent years. If you like Pillars then definitely do play WM.

Why would anyone buy anything if they don't even know what it is they're getting yet?

If you're at all concerned that an as yet completely unknown product might not be to your satisfaction, then don't buy it.

IMO, shelling out for DLC that hasn't even been announced yet is bad consumer practice that encourages companies to do bad things anyway.