IF 从句 可以这样省略吗

No.
从句主句主语是一致的 but one is "ask" another one is "tell"
can not take the "if" away.So "Play football well,and you can join us." is not good either,( Who?I can say "We play football well,and you can join us.)
Anyway this is a typical Sino-English.You should say like this:
If you are good at football,You may join us.
or If you like football,You may join us.