what you want to look at is getting a down quilt

a lightweight down jacket might keep you warm if its summer and the temp stays at 60 or so - otherwise, I think not.   One thing a sleeping bag does is protect your hands and feet and head from the cold - the areas of greatest heat loss - you'd lose that with a down jacket/pants


maybe something like this ? seems like a compromise - no zipper, but has a footbox

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or search backpacking forums for quilts