Chad,

Yes, I've done something similar. In addition to checking to make sure you don't invalidate the integrity of the existing pond bottom and making sure that the dirt used in the build-up has adequate clay (as opposed to muck), it is advisable to prepare the surface area on the dam for the new dirt. Remove grass, scuff up the soil, and make sure that the new dirt will readily mix and integrate with the existing dirt. Doing all those things should give you the result you are after.