however what is possible is a pool or even random user transacting can put one of their output UTXO's as not a spendable bitcoin key but instead a random phrase.
Another thing that is possible is a pool or solo miner to put a phrase in the input (extraNonce) of the generation transaction (As Satoshi did in the Genesis Block). Perhaps this is what the OP saw? If so, it's pretty meaningless. Those messages have no effect on the operation of Bitcoin.
there is no "extranonce" in a coinbase input.. what you mean is probably what you quoted me saying. where the output is not a valid address/signature but a space re utilised to form a message
00000050 01 01 00 00 00 01 00 00 00 00 00 00 00 00 00 00 ................
00000060 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
00000070 00 00 00 00 00 00 FF FF FF FF 4D 04 FF FF 00 1D ......ÿÿÿÿM.ÿÿ..
00000080 01 04 45 54 68 65 20 54 69 6D 65 73 20 30 33 2F ..EThe Times 03/
00000090 4A 61 6E 2F 32 30 30 39 20 43 68 61 6E 63 65 6C Jan/2009 Chancel
000000A0 6C 6F 72 20 6F 6E 20 62 72 69 6E 6B 20 6F 66 20 lor on brink of
000000B0 73 65 63 6F 6E 64 20 62 61 69 6C 6F 75 74 20 66 second bailout f
000000C0 6F 72 20 62 61 6E 6B 73 FF FF FF FF 01 00 F2 05 or banksÿÿÿÿ..ò.
01 - number of transactions
01000000 - version
01 - input
0000000000000000000000000000000000000000000000000000000000000000FFFFFFFF - prev output
4D - script length
04FFFF001D0104455468652054696D65732030332F4A616E2F32303039204368616E63656C6C6F72206F6E206272696 E6B206F66207365636F6E64206261696C6F757420666F722062616E6B73 - scriptsig
FFFFFFFF - sequence
01 - outputs
as you can see in a coinbase transaction it needs no 'signature' because it is a reward generated coin. and so you can put anything into the signature area.. this is not a "extra nonce" this is just a script/signature area.
some people also put in extra outputs, with 0 balance given to it because they want to use the address area(public key) as a message space
"extranonce" is usually deemed the utility of changing the time stamp by upto ~70 minutes(4200 permutations(12bits of timestamps 4bytes))
those diagnosing satoshi's mining habits of 2009 to work out which blocks seemed to follow satoshis mining pattern used the time stamps 12 bits as a term for "extra nonce." and thats what it has been associated as ever since.
becasue thats what satoshi first done in 2009-2010 as his extra nonce