You’re looking for an open return, not an open jaw (though you may want that too), and only if you purchase the most expensive and flexible fare (because you can change the return any time without incurring change fees and fare increases) can you practically accomplish this.

As for open jaw, the use of aa.com allows that by selecting Advanced search and using the Multi-city option. The open part of the jaw must generally be of less distance than the two ticketed sectors, among other rules. The fare is generally calculated on 1/2 of the round trip using the first flown sector plus 1/2 round trip using the second flown sector.

Examples of open jaw (and reverse open jaw) include SFO-SYD + MEL-LAX-SFO, or SFO-SYD-LAX.

This is another time one should take the time to select and read the Detailed Fare Rules thoroughly, as the composite fare will likely involve two (or more) fare types - and generally, the most restrictive rules apply to the entire ticket.