THHN through EMT - then convert to Romex?
You can transition wiring methods at a junction box.  Use the EMT wiring method up to the junction box (whole nine yards with fittings and clamps).  Then use the NM wiring method beyond it.  The junction box must remain accessible without screws, nails or demolition (other than the ones on the junction box lid, of course).  If you don't like the aesthetics of a blank junction box cover, then stick a receptacle there lol, or better, extend the conduit a little bit to a place you want a receptacle, and use an extra-large box so you have room for the splices. Then put a receptacle there fed off one of the circuits.  

You simply splice wire to wire, except for grounds. Normally there isn't a ground in the EMT conduit, the conduit is the ground.  Any junction box will have a #10-32 tapped hole and you fit a cute little green ground screw there with a pigtail.  You tie the pigtail to the Romex grounds. 

I get where you're a Romex guy and EMT/THHN feels alien to you.  I strongly recommend you "get acquainted with" this new wiring method.  It is actually rather versatile.  For instance, a conduit can take up to 4 circuits - or 8 with multi-wire branch circuits!  All circuits can share 1 ground wire, and if the route is entirely metal conduit and metal boxes, the conduit is the ground!  It's a fantastic system.   

That the attic will be finished is precisely why to think about extending conduit.  Conduit lets you alter the wiring later depending on your usage.  Find it's too hot and want a window air conditioner, but a 120V one would overload the circuit with your PC/laser, and you'd prefer a 240V unit anyway?  Easy peasy - pop a few box covers off, pull two more wires, instant dedicated circuit.  With stranded wire, pulling is easy. I do it all the time.