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.