Radiant works with raw .map files, which are compiled to BSP format for in-game use.
.bsp compilation is mostly a 'one-way-street', but you can technically decompile them, though they won't be like the original.
Things like lights will be missing (As they're 'baked' to a lightmap at compile time), and texture placement will be very wonky.
Also, some brushes will be deformed.
I'm running late, so I can't guide you through the decompilation process, but a quick google search for 'q3map2 decompile bsp to map jedi academy' should help.