to strengthen a squad : to get new players, so the group of players at a club is better.
The new Everton manager Roberto Martinez wants to do this in the summer transfer window:
“Everton must strengthen their squad this summer to build on the club’s recent success.”
http://www.bbc.co.uk/sport/0/football/22792389
The adjective ‘strong’ is also often used before ‘squad’: a strong squad
Brought to you by Football English, the vocabulary workbook for soccer.