Unfortunately, you cannot create a custom team in Madden NFL 12 for the PSP. The handheld version lacks a Franchise mode, which is typically where team creation and deep customization features reside.
The PSP release focuses on core gameplay, offering modes like Exhibition, Season, and various mini-games. While you can manage existing rosters and adjust strategies, building a franchise from scratch—including designing logos, uniforms, and stadiums—is not an option on this platform.
If team creation is a priority, you would need to play on a home console from that era, such as the PlayStation 3, Xbox 360, or the PC version, which include full Franchise modes with creation suites.