Yes, with mineral spirits or naphtha but the latter evaporates more quickly. I'd start with a 2:1 ratio of poly to varnish and adjust from there. 1:1 might about do it.

Just use some on a test piece before using it on any important.

Yes. I normally do a 1:1 with mineral spirits. It seems that the pre-canned version is a bit thinner than this, but what that means is it builds slower.

One important note is the flattener that makes the "satin" finish will precipitate out of this thinned mixture much faster. Stir before thinning, stir thoroughly after thinning and stir frequently during application or you will find your first area much glossier than your last area.

Thanks guys - just the info I needed.