This Java-based utility allows you to unpack, edit and repack an RPL/RPX Wii U executable.

If you intend to modify an RPL/RPX for game modding purposes you firstly have to unpack it. You can then edit the assembly in a hex editor. Then you should repack it again before trying to load the modified RPL/RPX with for instance Loadiine GX2.

Here.

Other than having Java 8 installed, no.

wiiurpxtool is used for packing. For unpacking, you have the choice between wiiurpxtool and rpl2elf.

0CBH0 for wiiurpxtool
Hykem for rpl2elf
BullyWiiPlaza for RPL Studio