Since you're not sharing any (reduced) document, i'll give you a highly simplified example.
Imagine that you have a facts table called Facts with Month_Key, Product and Amount. Add a temporary key to your facts table, consisting of the combined values of Month_Key and Product in each row, for instance Prduct & '|' & Month_Key AS Temp_Key. The facts table should be resident before executing the following statements.
// Create Increment to facts
Increment:
NOCONCATENATE DISTINCT Product RESIDENT Facts;
LEFT JOIN (Increment) Month_Key RESIDENT Facts;
// Concatenate increment to Facts but only the missing rows
CONCATENATE (Facts)
LOAD Month_Key, Product, 0 AS Amount
RESIDENT Increment
WHERE Not Exists(Temp_Key, Product & '|' & Month_Key);
// Cleanup
DROP Table Increment
DROP Field Temp_Key;
Best,
Peter
