Does anyone know how we can create an external reference from a string value
"pointing" to the external table?

Example
----------
A1=my_internal_table (content is text)
A2='C:\EXCELFILE.xls'!my_external_table (content is again text)

index(my_internal_table;2;1) --- works
index(indirect(A1);2;1) --- produces same result as previous
index('C:\EXCELFILE.xls'!my_external_table;2;1) --- works
index(indirect(A2);2;1) --- does not work (#REF!)