Get Custom HiDPI resolution rendering on non-Retina Macs
This forum post answers your question: InsanelyMac - Custom Retina HiDPI Resolution

It's pretty lengthy and complex, and probably not for someone who isn't happy with messing about with the terminal and plists etc.  In essence the steps involve messing with the DisplayProductID and DisplayVendorID values to fool the OS into thinking it's a HiDPI display.

It's not fair to simply plagiarise the OPs post, as this is from another user on another site, rather than a generic "use this program' style suggestion, so you'll have to follow the link rather than have it all pasted in here.