When I add charcoal to my smoker to keep the temp. where I want it I have always started the charcoal outside of the smoker and waited untill the coals were ashed over before adding them to the existing coals.

Can I just throw a few new briquets on to the existing coals and let them catch fire in the smoker?  or is it best to do as I have been doing and start them outside and add them when they are ashed over?

as long as you throw in just a few, and before the temp starts to drop you'll be fine.

I use lump charcoal and just pile it on top of the existing coals.

Quoted: I use lump charcoal and just pile it on top of the existing coals.

Yup.  Switch from briquettes to lump.  You won't look back.