Error running preprocessing with previously downloaded grib files

(trac ticket #298 by tcarioni)

When I'm running flex_extract, I sometimes get an error at etadot calculation :
Flex_extract error:

Command '['{fe_path}/flex_extract_v7.1.2/Source/Fortran/calc_etadot']' died with <Signals.SIGSEGV: 11>. 

I don't know where it comes from, and for debugging purposes, I would like to rerun flex_extract without re-downloading the grib files through mars request, since these files are already available.
Unfortunately, reading the (very good) documentation, I didn't see any way of running only the pre-processing phase of the raw grib files given by mars.
So I wanted to know if this feature could be added (or if it already exists but I'm not aware).
Thank you very much in advance.
Tristan Carion