Use Decrypt9.
1) Open FBI, go to Titles, and find the DLC for the game in which you want to turn into a CIA.
2) Make note of the title ID for that DLC.
3) Make a folder inside of your files9 folder called D9Game. Download the latest release of Decrypt9 by d0k3, move the Decrypt9.bin to your luma/payloads folder, and rename it to up_Decrypt9.bin. (if you have a payload assigned to up, assign it to another button).
4) Open Decrypt9 on boot by pressing and holding that button and powering on your system.
5) Go to Content Decryptor Options -> CIA Builder Options -> select either CIA Builder (SysNAND/orig.) or CIA Builder (SysNAND/decr.). It does not matter.
6) Use the arrow keys and the A button to find the title ID of the DLC you wanted.
7) Press A after finding the correct title ID and let it make the CIA.
8) You can find the generated CIA in your D9Game folder.