LEDs are not useful for growing cannabis. Many have tried; the result is a stretchy, light-starved plant. I can't see the point of adding any other lighting if you already have a 600HPS.

Yes, the replacement HPS lamps for security lighting at the hdwe are the same as SOME you can buy at a hydro shop- and will do just fine for flowering. However, at the hydro shop, you'll find horticultural versions of the 400HPS with a little extra blue chucked in to aid veg growth. Most well known is the Philips Son-T-Agro, available in a 430W which runs on a 400 ballast- but many makers sell modified spectrum HPS for hort use nowadays. You might find also the hort types at larger hdwe stores.