How do you copy a NAG parameter into a forcefield step-by-step with Ubuntu

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Hello,

I am new to this, and have used CHARM-GUI to create the NAG parameters (forcefield.itp file). How do I either add this to the GROMOS_54a7 force field OR copy the forcefield and then add the NAG parameters (rtp) to the dat file? I am having a hard time locating the dat and am unsure how to copy force field. Plus will this .itp file give problems when I add it to the parameters file since from what I have read is a rtp file? This is being done through a shell in Ubuntu for gromacs. I am normally a windows user, so it is new to me. If there is a step by step command option somewhere to be directed to that would be helpful as well. Thanks in advance

Heather