You can use certapp to add new certificates to an already existing certificate store.
The following example shows the generation of a software install (SWI) certificate store file, new_swicertstore.dat. This file is generated by augmenting the SWI certificate store represented by swicertstore.dat with certificate entries specified in extra_certs.txt.
> certapp --bswicertstore=swicertstore.dat --hswicertstore=extra_certs.txt --out --bswicertstore=new_swicertstore.dat