For programming and productions reasons, is there a way to merge or combine an existing bootloader and application hex image into a single file?

Yes. For Lyra, the below command can be used to merge an existing bootloader and application hex file into a single hex image with the help of the Simplicity Commander CLI. See also: What is the „Simplicity Commander“ and where can I get it from?

Syntax: C:\SiliconLabs\SimplicityStudio\v5\developer\adapter_packs\commander\commander.exe convert <BOOTLOADER-IMAGE>.hex <APPLICATION-IMAGE>.hex -o <COMBINED-IMAGE>.hex

Example: C:\SiliconLabs\SimplicityStudio\v5\developer\adapter_packs\commander\commander.exe convert LYRA-P_Bootloader_480-00184-R126.1.0.3.hex LYRA-P_Bluetooth_Xpress.hex -o combined.hex