You can do this by once loading the historical data from SAP and store the resulting table into a qvd.
SELECT .... from (Bex Query); // load the historic data
STORE Table1 into Table1.qvd; //store the data into a qvd file
The above needs to be done only once to create the qvd with the historic data. After that you can change the script so it first loads the qvd file, which will load very fast, and then load the monthly data from SAP:
LOAD * from Table1.qvd (qvd); // load the historic data
SELECT .... from (Bex Query); // load the monthly data
STORE Table1 into Table1.qvd; //store the complate table qvd file
The updated qvd now contains the historic data + this month's monthly data. Next month it can be used to load the historic data and the data of that month can then be added to create a new updated table (and qvd file).