Adding New Fields to Dynamics GP Tables


I am a newbie in Dynamics GP development. I have a requirement to add a new field to ReqMgmtLines table in GP DYNAMICS database. I dont have the full documentation on this implementation and I am not sure where the ReqMgmtLines is custom table. Is ReqMgmtLines a custom table or GP system table. Can I add new fields to GP system tables without causing problems? Also can I add new field to custom tables in GP?