constant deprecistion amount

My users have a few very old assets missed in FA that have been going through multiple round of modification and now nobody remembers the details. They'd like to register the current NBV as cost and be able to enter a fixed amount as monthly depreciation instead of using depreciation method to calculate. Can FA handle it? I am thinking of depreciation override, but this is manual process and they prefer not to do it since there are more than one asset.

Thanks,