Some radios can have the switches converted to proportional sliders/rotary controls, but some cannot.
Later sets especially tend to be more difficult, as they tend to use microcontrollers inplace of hardware encoders. Sets like the F14 are specifically designed to be flexible, so you can place a pot, slider, switch on any channel you choose- if only they were all built that way. Not sure why you want the navy version though with the twin sticks.
It's more money, but you might also consider a Multiplex Royal Evo set. Definitely a set that you will want to hang onto, with an extensive feature set that is ideal for your Engel u-boat. I understand they can be tuned to surface frequency, and Jeff and Dave Welch both use these radios on your side of the pond, so they should be able to offer more advice on that matter.