Is it even possible to do this?
I'm not clear on what you want to do.
In a game, the Land Claim Block will prevent any POIs it overlaps from being assigned to quests. It might suppress spawns, but I thought it was Bedrolls that did that. If you remove the Land Claim Blocks and Bedrolls then it should go back to being a POI.
Spawning a specific POI, like a Sawmill can be done via either the (1) World Editor for an existing map, (2) the Random World Generator with some configuration and some knowledge of Tiles, or (3) using Teragon and whatever features it might have including some knowledge of Tiles.
The World Editor lets you edit an already-generated map. It has its own quirks and several people on this forum know its tricks.
When it comes to Tiles, I happen to know that certain "Gateway" Tiles are often the target of the Trader POIs. If you were to arrange for the Sawmill to be the appropriate size and have the right tags, you can probably arrange for RWG / Teragon to nearly-always place it very close to a Trader, such as across the street. I would have to dig into what you wanted in terms of Tiles to use in your map to be able to advise more there. Then I could scout out those Tiles and the Sawmill POI and give you an idea of your chances.
A possible complication is that POIs cannot have player blocks in them, so you won't be able to prefill it with functioning workstations, storage chests, traps, and electricity.
Did any of that answer your question, or give you a lead on an answer?