Is it possible to make expansion teams select free agents?

Does anyone know?

Just do a custom league instead of expansion.

I tried it. Every time I add an expansion team to a division, there's an expansion draft where only signed players are selected. If I try to select by myself, there are no FA's in the player's pool.

I would say not, since free agents aren't eligible in real expansion drafts, and the feature has been designed to reflect that.

upload all the expansion team's jerseys and branding to your 2k account from your current myLeague.
Start a new MyLeague with custom league, use non expansion teams. Once in the mode download/ apply the team expansion team designs that you uploaded to your 2K account. Bam! done.

The idea was to have all 30 NBA teams plus 6 expansion teams. Because I unlocked a lot of FA's, I wanted them to be selected by the 6 extra teams, leaving the original 30 teams intact. So what I did was change the team type of expansion teams, making them NBA teams before starting the MyLeague. Of course they didn't have any players, so as soon as the season started, they all got filled randomly with players from the FA pool, so that's what I wanted. Probably we can do the same with the high-school teams, which is cool.

I know you can increase the number of expansion draft protections per team. I honestly don't know the upper limit, but it might be worth trying to put it at 13 for the teams to protect all the (active) players.