In theory you can do many things, but from practical experience I strongly suggest that you follow something like the following approach.
Source the complete(!) table into some intermediate storage. This can be a table or a file, that doesn't matter.
While extracting the data, you can calculate the "source_system" from the "claim" attribute using an Expression transformation (with the IIF function).
After you have extracted all records to the intermediate storage, truncate the table and load it from this intermediate storage.
Afterwards the table is complete.
Big disadvantage: during that re-load process the table is not accessible to anyone else.
Big advantage: this approach will most likely work much faster than UPDATE statements on the whole table.