Wolltest du Fußball spielen? Wenn Ja, mit wem?
Wolltest du Fußball spielen? Falls Ja, mit wem?
Those are both correct, the former being the somewhat less formal and the latter being the somewhat more formal form (in the South you may be perceived a bit snotty when saying "falls", in the North I'd say you are OK).
Note that although strictly that isn't the case (they're equivalent!) many people will imply an at least indifferent or positive-ish attitude with "when" whereas with "falls" the baseline assumption is rather negative.
That's probably because that implication is truly existent in the other use case of these two words:
Wenn wir ins Kino gehen, will ich Popcorn.
Falls morgen Fußball gespielt wird, komme ich nicht zum Spiel.
Here, the first sentence (weakly) implies that we're going to cinema (this is not much doubted), and when that happens, I'll be wanting popcorn (when, not if). The second sentence implies that it isn't certain at all whether or not we will play soccer tomorrow (but if we do, I won't come anyway).
Wolltest du Fußball spielen? Ob Ja, mit wem?
This one is, uh... strictly "correct" but very, very antiquated and awkward. Unless you are reciting a Psalm from the Bible or are a medieval author, you will be given a puzzled look if you say such a thing.
Wolltest Du Fußball spielen? Und ob!
Here you get the meaning of "well yes, of course, most definitively", and it isn't awkward at all. It may also express strong disagreement with a negative statement or question, such as "I didn't think you wanted to play...".