It is possible when you have teams at multiple levels.

Buy for example a ST at level 5 with quality 77%

Train 'till 100% using only attacking skills

Then sell him at one friend team at level 8 (and he would be only 40% at that level)

Train again 'till 100% using attacking skills

Sell him at level at one friend team at level 11 (and he would be only 40% at that level)

Train again using attacking skills

You would have a beast in attacking skills.