Change FF number mid-trip?

Hello,

I have a Flying Blue account and miles expire after 18months of inactivity. I am about to reach that limit so I need to make 1 flight with that FF#.

I am flying on a weekly basis SFO-ATL-CLT and CLT-ATL-SFO, with a 1hr connection each time.

Is it possible to use my delta SM# for SFO-ATL and use my FB # for ATL-CLT for a same PNR? I was not able to find my answer in recent threads. Thanks!