I am not familiar with the files produced by Insight, but if they are complete .psf and .crd files for your whole system all you need to do is:
* charmm script for protein+ligand
read rtf card name ...
read para card name ...
read psf card name my_system.pdf
read coor card name my_system.crd
If you have separate files for protein and ligand (eg, the files you get from Insight are just for the ligand) you need something like this:
* now with protein and ligand in different files
read rtf card name protein.rtf
read rtf card name ligand.rtf append
read param card name protein.prm
read param card name ligand. prm append
read psf card name protein.psf
read psf card name ligand.psf append
read coor card name protein.crd
read coor card name ligand.crd append
! use read coor pdb name .... if the coordinate files are in pdb format
The protein and ligand rtf and parameter files have to be compatible with each other (same atom types).