You cannot make a single threaded application multithreaded. It doesn't work that way. What you can do is cluster single threaded applications - ie run multiple copies of them simultaneously.
An example of this can be seen with node.js - A single threaded event driven java-script based environment. There are tools such as http://learnboost.github.io/cluster/ cluster which will manage several instances of a node cluster and balance the work across them.
By running multiple copies you will have a separate process for each instance which will then run on different cores.