My house has a 17 yr old natural gas furnace (ducted) but no A/C. If I want to add A/C is more efficient to add it onto the gas system or separately add a ductless heat pump system and just use it for cooling and still use the gas for heating?

The cost for more than 1 split, ductless systems would cost more than a central unit. You would be disappointed by excessive cooling in 1 room and lack in another. Although it depends on your sun load, insulation, windows, siding, people load etc. 2 tons would probably be adequate. You do not want too much AC as it does not allow enough time to dehumidify. Where is the heater located?

Adding A/C to the existing furnace would be the way to go if there is room.

Neither. Your 17 year old system is ready for replacement. New units are much more efficient. The cost to install a coil in your system would be considerable and that money would be better spent in a complete new system.