Adding new contrasts in FSL 1st level analysis without re-running model estimation

Dear experts,

I am using FSL for my task-fMRI dataset. I would like to add some new contrasts at single-run level. I wonder whether there is a way to do it without re-running the entire model estimation, which would takes an eternity.
I found an relatively thread on the fsl mailing list (2007) saying that there would be a “edit contrasts” button on the feat post-stats tab. However, I did not see it and suspect that that button does not exist in my fsl version (6.0.0).

Does anyone know whether there is a way to do it?

Thank you so much,
Yuan-hao