I do it all the time for minor topoffs. Rigged a garden hose off the gutter right before the downspout and put a little silicone seal dam just before the downspout hole so water hits the garden hose first. Hose goes into the skimmer. Heavy rain will overflow the little dam to the downspout. Water in my area (Northeast PA) is about as neutral as you can get. In some areas the rain is acidic.
Al