Need a Sub installation as my Home page

Hello Team,

I have integrated Question2answer script with WordPress. i have installed WP on root directory ( https://rosetta.numock.com/ ) and Q2A on qa sub directory successfully. Now i want ( https://rosetta.numock.com/qa/ ) as my default page on my site. So any one come to my site they should get redirected to https://rosetta.numock.com/qa/ but when they wants to register they should get registered with WP database.

Can any one help me ?