replace rear seat retractable lap belt?

I have a 1998 Chevy Blazer. Already have two car seats in the back seat for the kids -- but we're expecting a third child soon. The retractable lap belt in the middle of the back seat won't hold a third car seat, so I was wondering if it's possible to replace it with a non-retractable belt? Otherwise I fear we'll have to sell and join the minivan crowd -- yeech!

Thoughts ???