You can make any team have full stats by editing each players stats
and upgrading them to 99. However, be warned; if you start with this
team in manager mode, your salary will be high. Make sure to cut
their contracts down as soon as possible.

To start with a five star team immediately in career mode, go to the
team management screen. Put the bad players in the first eleven so
that your team will be a four star team. Then go to career mode.
Your team should be there. Save your career, then go back to the
team management screen. Put the first eleven players back, save it,
and you will have the normal team on friendlies and your team on
career mode.

Use the following trick to be any team desired in career mode
without buying better career teams from the fan shop. First, go to
team management and choose a four and a half or five star team. Put
all their worst players into the starting line up. They need to go
down to about four stars or less. You can then use them in career
mode. When you have begun your career you can change your team back.