That sentence is confusing at best. Is there any context to it? Regardless, I'll give it my best shot.
Let's say you have an alkyl halide compound that you believe to be contaminated with some alkenes. To selectively convert the alkyl halides to alkyl azides, you would just perform an azidation (is that the right word?) reaction, which replaces the halide group with an azide group. You can then use a solvent like DMF, as suggested here, or perhaps ethyl acetate, to extract the alkyl azides.