Is there a way in FMC to copy access rules from an ACP and paste those in another ACP which is already loaded with access rules and applied to an FTD. Actually, will have to 2-3 migrations of ASAs to FTD but at different time frames. So, at last need to have all consolidated as one ACP. Tried inheritance policy option but can't modify the copied rules.
It's not supported - even in FMC 6.6.
If you use CDO management, it is quite easy to do. However CDO cannot (currently) manage a device that is also FMC-managed.
CDO uses the API to interact with managed devices along the lines @Francesco Molino was referring to.