What do you mean?
- Custom graphics? Just same as with SMBX and LunaLUA-SMBX
- Add more elements? Edit configs/<config pack>/*.ini to change number of elements and then in the "items" folder you will find set of INI files per every element. Copypaste one of them into new ID and change properties as needed. Note: customized config pack will be shown at you, but to deploy for people you must also deploy customized config pack
- Draw extra stuff? PGE Engine currently lacks user level scripts, but you able to modify global level script of config pack. Be careful, is not same as LunaLUA and partially not documented or you need look for documentation into PGE Engine's source code. But if you making level for LunaLUA-SMBX, you are able use lunadll.lua / luna.lua script in the level custom folder to deal with custom rendering stuff.