Control.com - Nerds in Control

Can I simulate 4-20ma signal to a loop powered device such as pressure and level transmitter in the field going to the DCS? (even if the two wire of the transmitter has 24VDC supply from DCS)

Yes, transmitter simulation is standard on most calibrators.

To expand on Roy's answer, many field devices called "calibrators" have the ability to "source" or "simulate" 4-20 mA signals. These calibrators are usually very similar to many multi-function digital voltmeters. If one of these calibrators is put into "source" mode, it will provide the 24 VDC (nominal) "power" as well as regulate the current between 4 and 20 mA for use in checking or troubleshooting circuits.

When a calibrator is put into "simulate" mode, it must be powered by a 24 VDC supply and will then regulate the current between 4 and 20 mA.

At least that's the terminology and configuration of most Fluke-manufactured calibrators and advanced-function voltmeters I have had the privilege to use, and some others as well. I'm sure other manufacturers have similar, or even the same, terminology for the same functions for their products.