Here we go!
The “Date&Week” Metanode
The “Read” Metanode
Let’s see what’s going on in this workflow:
The Metanode reads your excel sheets (wk1 and wk2) and finds first date of wk1 and last date of wk2 (and the weeks) and calculates the number of weeks (although it’s obviously 8 (7 for wk2 sheet and 1 extra week from wk1) but I wanted everything to be calculated automatically, so if the data structure is changed you can still use this workflow).
Then we have the first “Create Data&Time Range” node which creates dates based on the starting date in your sheets and the number of the weeks (in weekly intervals) and a counter to create week numbers, starting from the starting week number. Column Expressions converts week numbers (produced by counter) above 52 to start from 1 again.
At the bottom of the workflow we read data from the excel sheets and join them (first week of wk1 + all weeks from wk2). Then unpivoting (Thanks to @Iris 
) “base forecast” and “GR (SU)” and joining them.
And finally adding the date and week columns and filtering based on the execution date.
Here is the workflow, dedicated with love! 
Merge&Transpose.knwf (223.7 KB)
Best,
Armin
PS: The week 1 for 2019 in your sheets has different values. Anyway, I only took the first week of wk1 sheet (week 51) and the other weeks are taken from wk2.