Nothing is as easy as it initially appears.

To do this correctly, (now I'm just thinking off the cuff here), your L6 would have to be designated as creep - so we would have to introduce a new stress type - say "CRP" in the drop list. That means everywhere we use or check for the stress type we have to make allowances for the new "CRP" designation.

Then in the allowable stress routine, yes we would add the computations to determine "fcr" - that part is easy.

There are likely other issues to address, but I haven't thought things through.