If you attach a statement to an account on your credit record saying this account does not belong to you can this account be used for a FICO Score?

It is not possible to "attach a statement to an account". It is only possible to place a consumer statement on your report. This is important distinction and should answer the remainder of your question. In general, your statement, which may refer to a certain disputed account, shows to all who access your credit. But the fact that it is separate from the account itself means that the account would be used in the calculation of your credit scores.