Can u explain a little more the Situation you described it?.
By Infrastructure you mean the Hardware/Software Infrastructure that allows the ESB to function, or the Reference Architecture applied to the components it contains?.
In which technology/platform the "New System" make avaiable the resources you must access?.
What Software Application are you using as ESB?.
You can use transformation for One system to another system, OSB provide different transformation standards and different protocols to resolve this.
for MainFrame system you can use tuxedo transport. if your both systems using XML format in any format then you can go via Xquery or XSLT transformation.
if you required more information SO please share detailed information about application and Infrastructure,