Disregard my previous answer - it was meant for the question about appending time steps.
For this, you need the @ASN regridding transformation. Ferret's regridding is one of its most powerful tools. When there are two calendar time axes, it takes the size of the time step and the time origin, and translates between two axes. So data on an axis starting in March 2010 which is measured in days since January 2000 can be regridded to an axis in seconds starting in December 2014.
Because your original data is not a calendar time axis, you the user can use @ASN to tell Ferret that this is time measured in days starting at January 1 2010.