are you wanting to keep the same ghostly feel to your images ... ?
a flash may change all that completely and introduce shadows and contrast where you had none before .

what kind of a flash do you have? can you choke it down and reduce its output and direct the light up or to the side?

does it have a pc/cord or only a hot-shoe ?

your camera is "sync'd " where you see shutter speed with an x or its red ( 60? 125? 250? )
so you can shoot with your flash at that speed or slower

i you can effectively paint with light, with your flash if you want as well... depending on
how long your exposure is ( seconds? minutes? ) you put your flash on low power
and get some tracing paper to mute it even more ... and burst light on the parts of
the scene that you want to have more light.

you might also try developing your film with a different developer to reduce the grain
that might help enough so you won' t have to even bother with a flash ..

good luck !
john