2 Options:
1) Wait for an upcoming mod called the "Player Switcher" OR
2) Go into your STREAM folder.
A) Open the WORLD.IMG (Download IMG TOOL, google it)
B) Find the desired character model you want, get their .NFT and .NIF file (.NFT is the mesh and .NIF is the texutre, if you do not know what those are do not worry)
C) Find the desired thing you'd like to replace the character you want with (Must be Jimmy's clothing)
D) Usually the black ninja is used. So, extract the .NFT and .NIF of the character you want (Not the one your replacing, the one you want to be) to a folder.
E) Make copies of those to match the number of files the ninja has (Or what you are replacing)
F) Look at the names of what your replacing (Again, probably ninja) which you found in step C. The black ninja is the SP_Ninja_FT, SP_Ninja_H, SP_Ninja_L, SP_Ninja_T (Totaling 8 files with both the .NFT and .NIF of them)
G) Rename each of the files of the desired characters with the name of the ninja (You should of copied them to match the 8 files the ninja has/or whatever your replacing in step E)
H) Delete all the ninja (Or what your replacing) files inside of the .IMG.
I) Go to "COMMANDS" at the top and add the files of the character you want that you renamed to the ninja.
J) Rebuild the archive in "COMMANDS"
Common issue: Your player is silver, this means he wasn't textured.
Fix: Go into the .IMG again and find the ninja files (Or what you replaced) and assure there aren't more .NIF or .NFT files then there should be (If ninja there should be 8, 2 of each letter (The letters are the FT, H, L, T))
Remember to always rebuild the archive.
Hope I helped.