Dissolve the polymer in the appropriate solvent (that doesn't dissolve the fiber too!) and pass the tow of fiber through a bath containing that polymer solution. Allow to dry or bake dependent on the fiber and polymer.
If your material is included in a polymer process, like extrusion as an example, you will want to add a wetting agent suitable for the type of fiber you have. You should contact Struktol (Cleveland OH area), they have several products that will likely work for you.