Yes, it is possible.

You would put the text in the pk-script of your transaction output(s).
The most common method is to use OP_RETURN opcode, followed by the data (text message) you want to notarize.

Just remember that if you put too much data after OP_RETURN, you might have problems getting the transaction confirmed.
But the blockchain protocol itself allows a transaction to be as big as 100KB and you can fill it with a text almost entirely.
Apart from the limitations inside the official software, miners should not have problems including such a transaction into a block, as long as you'd pay them a sufficient fee.

A cheaper way however would be to only embed a hash of your message into the blockchain, while the message itself would be broadcast via some other channel.