Sure, you could easily do that. Don't rack up any flocculated yeast, though- the idea is to get it OFF of the yeast.
If it's already lagering, it's done fermenting, so you're all set. Let it lager for the allotted time, then carb and serve- all in the same keg.
No need to vent any pressure. Any residual CO2 will just help carbonate the beer.
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