Depends on what you mean by a HART system. What defines it as a HART system?
A. Do the analog inputs have a HART modem that reads one or more HART variables?
B. Does the HART AI act as a passthrough for an asset management system to use HART to configure the field devices?
C. Are the HART devices on a HART multiplexor?
D. Are the HART devices all on HART multidrop network with HART master that polls them one at a time?
E. None of the above but the field devices can be configured and troubleshooting done by a HART communicator of some sort?
There is no device that 'adds' HART to a non-HART device. So if that's what you're looking for try, solicit an opinion from someone who is less straightforward than I am.
Situation A: Your non-HART device's 4-20mA will work on a HART enabled AI but your device will not provide any HART data, only the 4-20mA signal.
Situation B: Your non-HART device will work on a HART enabled AI but without HART the asset management software can not talk to the device.
Situation C: Your non-HART device's 4-20mA signal will generally work with HART multiplexor but again, no HART data is available, only the 4-20mA signal
Situation D: Your non-HART device will not operate at all on a HART Multidrop network.
Situation E: Your non-HART device will work as well as any non-HART device with a 4-20mA output, but of course, it can't be configured with a HART communicator.