Hi!
According to ADaM IG section 2.3.2 on page 9: “The BDS is flexible in that additional rows and columns can be added to support the analyses and provide traceability, according to the rules described in Section 4.2.”
Therefor the answer for your question is “Yes, it’s allowed”.
I personally like your approach and always keep original SDTM/8601 --DTC variables in analysis files, because it’s great for 1. backward traceability and 2. for simple imputation as you described.
Regards,
Sergiy Sirichenko