You can only place a meeple on a tile you just placed. The Princess & Dragon expansion provides an exception to that rule.
As Gendolkari already quoted, the rules in the vanilla game are clear:
The player may only play 1 follower on a turn.
The player must take it from his supply.
The player may only deploy it to the tile he just placed.
However, in the "Princess & the Dragon" expansion, there are 6 magic portal tiles:
If you draw one of those, instead of your normal meeple placement, you may place your meeple on any other already placed tile. You follow all the other rules for meeple placement. In addition, you may not "teleport" your meeple into an already scored feature.
Note that a farm is not scored before the end of the game, so even when a farm is completely enclosed and can no longer expand, as long as it's unoccupied, you can place your meeple into it.
From the Princess and Dragon rules:
When a player draws and places a magic portal tile, he may place a follower on this or any previously placed tile. When doing so, he must follow all other placement rules, for example, not on a feature with another follower. In addition, he may not place his follower on an already completed feature.