would ''bagaimana kamu'' mean something similar to ''apa kabar''?
Bergaimana ~ how about it
Bagaimana ~ how
Bagaimana is shown as meaning HOW and WHAT? when do we use APA?
apa = what
Depends on how you mean it.
Bagimana is pronounced bâgēmânâ not bagaimana
This is because i sais ē in B.I.
Is it possible to add "ada" ? Bagaimana ada(lah) mereka ?
No. Ada means "to exist", while adalah means "to be (equal to)". Neither meaning fits the sentence.