You won't be able to successfully bend, twist, contort or interpolate the rules of evidence or statutory law to fit those weird religious prejudices you hold.
In the instance you reference as federal rules of civil procedure, documents , ancient or not, falling under the definition of hearsay, will again under the same rules of evidence, remain such until the authenticity of them can be established.
The alleged Josephus text immediately defines itself as hearsay. The authenticity of the alleged Josephus text cannot be established for many and obvious reasons. Therefore no exception and remains as hearsay.
We are not talking of Josephus, we are talking of Christian forgers who by interpolation alter original text which itself stands only as hearsay in the first place.