Building Below bedrock.

For some initial information I'm in minecraft 1.21.4, running a vanilla server with the techtonic generation datapack. I want to be able to build around 100 blocks below bedrock and Im pretty sure you are able to allow this with datapacks although I have no clue where to start. There are some other packs I found that did this but replaced the bedrock or messed with the other heights, I want to keep the vertical height at the 620 set by tectonic while having bedrock at -64 but allowing players to place for around 100 blocks below bedrock similar to story mode. Could someone help me set this up for my server. It would be greatly appreciated.