you can just quote the post like your about to reply to it then just copy the text body, the bit in the " [quote] .... [/quote] " box and go to "User CP" and put it in your signature and it should show up as a direct quote from that poster.
Someone posted something on my 'I may be heartless saying this' thread about the Shannon twins from CBB and I wanna know how I can make that particular post as my signature, I've seen others do it.