Mounting long reach center pull brake on Marrakesh (or similar disc-braky) frame? I wanted to build a  commute/touring bike using a existing parts.  I'm considering Salsa Marrakesh or similar frame that's designed for disc brakes.  I'd love to use rim brake 650B wheels I already have.  Is it possible to mount long reach center pull brakes on these frames.  

I see holes both on fork and seat stay, which are meant for mounting rack.  Wondering why can't I mount center pull brakes there.
As Sheldon Brown notes, 650B rims are 19mm further away from the brakes than an equivalent 700c. Given that your frames were designed for disc brakes in the first place, you will have some difficulty in finding rim brakes with enough reach. You might need an unsafe hack -- especially if you are using fenders. 

Furthermore, your brake cabling is set up for disc brakes so you'll need to get new braze on guides or use ugly zip ties. 

Most people would wonder why you'd go to such lengths to have worse brake performances and worse looks in order to keep your hubs. There are a number of great looking disc brake hubs now available. If you found one with the same spoke count, you could keep your rims.