CHARLOTTE, North Carolina (Reuters) - Wells Fargo & Co may have to buy back an additional $1.8 billion in toxic mortgages from outside investors on top of claims it already received, the fourth-largest U.S. bank by assets said in a securities filing on Friday. (http://feeds.feedburner.com/~ff/reuters/topNews?d=yIl2AUoC8zA) (http://feeds.feedburner.com/~ff/reuters/topNews?i=1hH2Js5nf2M:Co9sfymVtoU:V_sGLiPBpWU) (http://feeds.feedburner.com/~ff/reuters/topNews?i=1hH2Js5nf2M:Co9sfymVtoU:-BTjWOF_DHI) (http://feeds.feedburner.com/~r/reuters/topNews/~4/1hH2Js5nf2M)