Here is an explanation for how to copy a superstructure drawing from one of the historical carrier templates into your game so you can use the drawing for your own ships, including those converted ships that have no carrier drawing. A word of warning, this is not an official, sanctioned process and involves mucking around in your ship save files, so the usual warning applies - back up your files before you start messing around with them.
Ship superstructure drawings are stored as part of the ship file. Each ship in the game (and each refit version of each ship) is stored in the RTW2/Save/Game# directory. Within this directory you will find each ship file stored as a file titled: "ShipName.#d". If the ship has been rebuilt the format will look like this: "ShipName (R year).#d". For instance: "Amalfi (R 1923).90d".
The ship files are simple text files and can be edited using any text editor such as Notepad. (Don't use a program like Microsoft Word because it might add special characters that will break the file.) Again a reminder, backup your ship file before you try this.
Open the file in a text editor and search for the line that reads: [Funnels]. A few lines below the [Funnels] section you should see a section titled [Superstructure0], which marks the beginning of the list of points that describe the superstructure drawing of the ship. The [Funnel] section and everything below it, all the way to the end of the file, describes the superstructure drawing of the ship. The [Funnels] and [Superstructure#] sections are always a big chunk of a ship file, typically occupying 2/3 to 3/4 of the total file. This is the section that you will be replacing when you copy a carrier superstructure drawing.
To copy a superstructure drawing (including the funnels), simply copy this section (starting with the section title [Funnels]) out of a carrier template file. Now select the same section in the ship file that you want to modify and paste the copied section over the corresponding section in your ship file. Make sure that you completely replace all lines in the old [Funnels] and [Superstructure#] sections with the section copied from the carrier template. (If the design does not have any funnels the [Funnel] section will be missing. In that case start with [Superstructure0].)
You can make this change while the game is running with no adverse effects, however the new drawing will not show up until after you have saved the ship design. Once the ship design is saved the new superstructure will show up and you will be able to freely edit the superstructure drawing the next time you refit the ship. The same applies if you are designing a new ship from scratch. You will have to wait until you save the newly designed ship so that a ship file is created by the game before you can swap out the superstructure part of the ship file. However, as soon as this file is generated by the game you can modify it while the game is still running.
One thing you need to keep in mind is the total displacement of your ship compared to the carrier template you are planning to use. You will want to select a drawing that is of a ship that is roughly comparable in size. If your ship drawing is too large the bow and stern of the ship may be cut off when displayed in the game and the size of the ship in battles will be visually wrong. If the size of the ship template used is too small then a lot of the default hull drawing will show up around the superstructure drawing. (Because the templates were drawn quite early in development some of these drawings will always be too large.)
Below is an image showing what happened when I copied a carrier template for a 26,900 ton carrier (Kaga in 1928.20d) for use on a 5,000 ton seaplane carrier. The new image is inappropriate because it is much too large for the small ship. To assist you in this process I have included below a list of the available ship templates that also lists their original displacement.