Can I make a pH 7 buffer from phosphoric acid and sodium hydroxide?
You should be able to do this with little trouble.  The only caveats:


Make a stock solution of phosphoric acid and also of NaOH.  When you add them to each other to make your buffer, heating could be a problem, much moreso than if you were using the mono- and di-basic sodium phosphate salts.  So you may want to cool on ice when mixing these compounds.
You can calculate approximately how much NaOH and phosphoric acid to mix in order to get a pH 7 result, but it's best to use a pH meter to verify that you've reached pH 7.