In addition to Gysberts comment:
You can als use IF statements in the script. So in in that case you can manually configure the script to load QVD data or data from the source. Like this
LET vLoadQVD = 1;
IF FILETIME('Path\QVDNAME')>0 AND $(vLoadQVD) 1 THEN
LOAD * FROM QVD;
LOAD * FROM Source;
STORE Table into QVD;
I also made an escape for the case when the QVD is missing.
Depending on your full requirements you may be able to achieve what you need by using sub routines.
If you wrap each of your loads in a sub function, when you then call your sub routines you can add an if statement that only loads the required tables.