Merge down? As in Photoshop? I'm not sure what you're asking here. In FCP if you want to combine several tracks into a single track, it's called nesting and it's done simply by selecting the tracks you want to nest and hitting OPT+C. It'll put the selected tracks into a new sequence and edit that sequence into place in the current sequence. It's not technically a "merge down" function as it relates to Photoshop because it's non-destructive. But it is often used as an organizational tool if you have lots of tracks and you want them grouped for whatever reason. Nesting preserves any transparencies you have.
Keep in mind that nested sequences can be troublesome in the realm of media management EDL/XML exporting as well as some other areas so be careful.
If you're referring to Photoshop, you can stamp layers to a new layer. Typically when I'm importing a photoshop file into FCP, I create my master PSD and then an FCP "friendly" PSD which has all the layers merged that I don't require to be independent in FCP. Just make sure you aren't needing to make constant changes to the original PSD or else you find yourself doing double the work in Photoshop.
Andy
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