How to create a program that copy and paste between instances of excel???

Hi,
I am new to programming and wanted some advice.
The problem that I am facing is as follow: I have a simulation software that export to Excel. I am able to automate the export function of the simulation software of the via Excel VBA (call this ExcelA.xls). However, when the software export, it export to a new instance (ExcelB.xls). Thus I cannot do simple programming to activate in ExcelB.xls.

What I want to do is copy the data (from say Sheet1) from this new instance (ExcelB.xls) and paste it into the (say Sheet1) of ExcelA.xls of the old instance.
And finally close the new instance ExcelB.xls without saving.

Does anyone know how to approach this problem?

some examples would be great

Thanks ahead

The new instance does not recognize the old workbook (excelA.xls). I also wanted to do this in an automated way, so that the user doesn't have to do it.