Well, there is one way of doing it.
First off, download either iExplorer or i-Fun Box to your Mac/PC (yes, this requires a computer to do it). Second, plug in your iPod, and open up the program you downloaded (being iExplorer or i-Fun Box, whichever you downloaded). Next, you will need to go to Apps, find Pixel Gun 3D, go to Documents, and through there, you should see a bunch of skin files.
Next, simply add a skin in there (make sure the file extension is set to .file, otherwise it won't be read, and make sure the skin file name follows the pattern, such as skin_multi_2, or something like that).
Then, Save Changes, disconnect your iPod, boot up Pixel Gun 3D, and they should show up. Please note however, depending how you named it, it will either show up replacing a skin in the purchase selection, the default selection, or in the Minecraft skin maker selection.
It's complex, I'l admit, however this is the only way to do it for iOS right now (unless you have it jailbroken with iFile, in which case do the same process, but through iFile). Sorry.