When is it possible to reclaim land in Freeciv by transforming ocean and lake tiles into swamp?

Terrain transformation requires the following two conditions to be met:

1. The tile is surrounded by at least N land tiles.

The number N is determined by the current game's ruleset, specifically the option ocean_reclaim_requirement = 30%. By default, since the game defines that each tile borders 8 other tiles, this is equivalent to needing 3 adjacent land tiles. In order to transform an arbitrary tile in the middle of the ocean, you would need to transform at the minimum a 3-tile wide bridge of tiles towards it, for example in a pattern like this:

2. The tile needs to not be in the permafrost region of the map. These are any tiles that are frozen at the start of the game. So snow or ice tiles.

There's another map parameter thaw_requirement that sets this. By default the value is 101, which does not allow thawing or freezing tiles via terraforming.

Also see the relevant wiki page, or browse the code over at github: the relevant section is the function terrain_surroundings_allow_change in /common/map.c, and the function can_reclaim_ocean.