I think I can create a HEX file for ROM by using this HEX converter.
Why is a ROMization processor (romp850) tool provided?
You may have misunderstood the application of this tool.
The HEX converter is used only for converting the object format into the HEX format and has no direct relation to ROMization referred to in the CA850.
You may have confused conversion to the HEX format with ROMization, because the HEX format can be read by the ROM programmer.
ROMization in the CA850 involves setting the initial value of variables with an initial value, storing in ROM a program to be executed in the internal RAM area, and copying it from ROM to the actual execution area (RAM) when the program is started.