As I understood the question, the OP has a cassette with 10-speed spacing, but it's missing a cog, so there are only 9. He wants to add a 27t cog to the 10th position (closest to the hub), which would be fine, if he can find a loose 27t cog and has an appropriately wide spacer.

However, as 70sSanO points out, if the missing cog is the first position (closed to the dropout), it may not work because those are specially shaped to grab the splines at the very end of the free hub. They also have scoring to help lock in the lockring.