Is there a proper way to backdate training forms like the DA 705 and DA 7801 when the data is available in DTMS?

Any form that has to be signed should not be rubberstamped even if DTMS is correct. It's fraud plain and simple even if it's just one of a dozen DA 705's you have on a Soldier.

They can be annotated on a DA 705 (unsigned and each block un-initialed) with a memorandum for record attached of the source of the scores and maybe a print out from DTMS. That is how I would go about it.

For future reference, both of those forms belong in a Soldier's iPERMS, and they can and should be uploaded in DTMS for easy retrieval.