Conversion of multiple DCD files into trr file for gmx rotacf calculation

GROMACS version: 5.1.5
GROMACS modification: No
Hi all,
I am trying to calculate rotational correlation function of a protein, by using long trajectory length of ~ 100 ns (100 files each of 1 ns) DCD files genarted through NAMD followed by converting them into a trr file. For that, I am using catDCD and it is successfully generating trr file.
But when I am using this trr file for gmx rotacf calculation, it is showing erros as:::
Failed to read magic number of trr file, this is not a trr file

If anyone has any suggestion on how to get successful results on converted data (from DCD to trr), it is highly appreciated.

N.B.:: For a single DCD file, by loading it to vmd and thereby saving as trr is working fine (generating successful results for gmx rotacf), but saving 100 ns long trajectory in a single DCD file will be expensive in terms of memory.

SO , how to convert multiple dcd files into a gromacs readable trr file?

Thanks and regards