Definitely. I usually soak a muslin bag in vodka for a while to sanitize it a bit, ring it out, load it with hops, and drop it in. It's worked great for me every time.
That's one of the beauties of kegging. You can make modifications to your beer after it's "done." Recently I had never used...
Your comment about your FG not getting as low as you'd expect makes me wonder about your yeast. Fermenting at 75 could lead to undesired flavors but shouldn't really affect your attenuation.
Any chance that the yeast in the kit is old? Depending on the shop those kits can sit on the shelf...