If you can put the skin on a web server, the easiest way to import it into Plus42 is to use the skin loader: Main Menu → Select Skin → Load...; enter the URL of the directory where the gif and layout files are stored, then press Go; then select and load the skin like you would when loading a skin from my web site.
Alternatively, email the skin files to yourself; on the phone, save the files to Downloads; and finally, in Plus42, use Main Menu → File Import & Export → Import Files to import the files into Plus42's private storage.
Note: You could use the SkinMuncher tool to create the skin. This is what I used to create the PortraitShort, LandscapeLeft, and LandscapeRight skins from PortraitTall. The tool is in the free42util repository at SourceHut. It requires a Java JDK to compile and run.